Set.isSubsetOf in JavaScript: check a shift against required skills

Explore Set.isSubsetOf through this working example: check a shift against required skills. Change the input and inspect the result before using your own data.

A shift needs first-aid and check-in coverage. The assigned team offers both plus translation, so having extra skills should not make the coverage check fail. A subset test answers whether every required skill exists somewhere in the available skill collection.

The result is true: first-aid and check-in both appear in available. Translation is an extra capability, not a reason to reject the team. This direction matches the scheduling question because required is the collection whose every member must be covered. Looking from available toward required would incorrectly ask whether the team has no unrequested skills.

Check a shift against required skills: source values beside the original result.
Check a shift against required skills: source values beside the original result.

Try the result before changing the rule

Edit the JSON and select Update preview to rerun the same expression. Original data restores this starting sample; Changed data supplies another preset. The operation runs locally in the example, so trying a value does not send an invitation, change a product, or save a setting.

Live exampletry it here, then copy the code
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<!doctype html><html lang="en"><head><meta charset="utf-8"><meta name="nos-demo-layout" content="responsive-lab"><meta name="viewport" content="width=device-width,initial-scale=1"><title>Check a shift against required skills</title><style>*{box-sizing:border-box}body{margin:0;background:#edf1f0;color:#172d36;font:15px/1.5 system-ui,sans-serif}main{max-width:1040px;margin:auto;padding:24px}header{display:flex;justify-content:space-between;gap:12px;align-items:center;margin-bottom:20px}.eyebrow{font-size:11px;font-weight:800;letter-spacing:2px;color:#55716b;text-transform:uppercase}h1{font-size:clamp(23px,4vw,34px);line-height:1.15;margin:8px 0}h2{font-size:16px;margin:0 0 12px}.tag{border:1px solid #bfceca;border-radius:99px;padding:6px 12px;font-size:12px;white-space:nowrap}.panel{background:white;border:1px solid #d4dfdb;border-radius:18px;padding:20px;margin-bottom:14px;box-shadow:0 9px 30px #1d393407}.grid{display:grid;grid-template-columns:1fr 1fr;gap:14px}.result{background:#17352f;color:#f7fffb}.result h2{color:#c9ded5}.cards{display:grid;gap:10px;grid-template-columns:repeat(auto-fit,minmax(135px,1fr))}.card{background:#f2ffc2;color:#17352f;padding:15px;border-radius:12px;min-width:0;overflow-wrap:anywhere}.card small{display:block;color:#536f60;font-size:11px;font-weight:700;text-transform:uppercase;margin-bottom:4px}.field{display:block;margin:8px 0}.field small{margin:0 0 3px;text-transform:none}.value{font-size:15px;font-weight:650;white-space:pre-wrap}.hero{font-size:clamp(28px,5vw,46px);font-weight:750;line-height:1.2;overflow-wrap:anywhere}.muted{color:#587169;font-size:13px}.result .muted{color:#bed7cb}pre{margin:0;white-space:pre-wrap;overflow-wrap:anywhere;font:13px/1.6 ui-monospace,monospace;max-height:220px;overflow:auto}.toolbar{display:flex;flex-wrap:wrap;gap:8px;margin:12px 0}button{background:#17352f;color:white;border:0;border-radius:10px;padding:10px 14px;font:600 13px system-ui;cursor:pointer}button.secondary{background:#edf3ea;color:#244337;border:1px solid #d0ddd0}button:hover{filter:brightness(.94)}button:focus-visible,textarea:focus-visible,summary:focus-visible{outline:3px solid #749919;outline-offset:3px}textarea{display:block;width:100%;height:126px;border:1px solid #bccfc5;border-radius:10px;background:#fafcf9;padding:12px;font:13px/1.5 ui-monospace,monospace;resize:vertical;color:#15362c}label{font-weight:650;font-size:13px}details{margin-top:16px}summary{cursor:pointer;font-size:13px;font-weight:650}#status{min-height:20px;font-size:12px;color:#526f60;margin-top:8px}.error{color:#ffcfbf}.footer{font-size:12px;color:#627b72;margin-top:16px}.empty{font-size:18px;color:#c7dacd}.source-card{background:#f4f7f3;border:1px solid #dbe4d8;border-radius:10px;padding:12px;overflow-wrap:anywhere;white-space:pre-wrap}.source-list{display:flex;flex-wrap:wrap;gap:8px;max-height:170px;overflow:auto}.source-list .source-card{flex:1;min-width:100px}.code{background:#f5f8f2}mark{background:#f2ffc2;color:#183a2d;padding:2px 5px;border-radius:4px}@media(max-width:620px){main{padding:15px}.grid{grid-template-columns:1fr}.panel{padding:16px}header{align-items:flex-start}.tag{display:none}h1{font-size:25px}.cards{grid-template-columns:repeat(auto-fit,minmax(125px,1fr))}.source-list{max-height:120px}pre{max-height:150px}}</style></head><body><main>
<header><div><div class="eyebrow">JavaScript • Set isSubsetOf</div><h1>Check a shift against required skills</h1><div class="muted">Change the data. See the actual result.</div></div><span class="tag">Interactive prototype</span></header>
<section class="panel"><label for="input">Input data (JSON)</label><textarea id="input" maxlength="10000" spellcheck="false"></textarea><div class="toolbar"><button id="run" type="button">Update preview</button><button class="secondary" id="original" type="button">Original data</button><button class="secondary" id="changed" type="button">Changed data</button><button class="secondary" id="edge" type="button">Empty / zero case</button></div><div id="status" role="status" aria-live="polite"></div></section>
<div class="grid"><section class="panel"><h2>Source after the operation</h2><div id="source" class="source-list"></div><p id="source-status" class="muted"></p></section><section class="panel result"><h2>Returned result</h2><div id="result" aria-live="polite"></div></section></div><details class="panel code"><summary>Inspect the exact values</summary><pre id="inspect"></pre></details>
<p class="footer">Local sample data only. No server request, account, or installation is needed.</p></main><script>
'use strict';
const initial = "{\"required\":[\"first-aid\",\"check-in\"],\"available\":[\"first-aid\",\"check-in\",\"translation\"]}";
const alternate = "{\n  \"required\": [\n    \"first-aid\"\n  ],\n  \"available\": [\n    \"first-aid\",\n    \"check-in\"\n  ]\n}";
const empty = "{}";
const input = document.getElementById('input');
const output = document.getElementById('result');
const source = document.getElementById('source');
const status = document.getElementById('status');
function transform(data) { return new Set(data.required).isSubsetOf(new Set(data.available)); }
function readable(value) {
 if (value === undefined) return 'undefined';
 if (typeof value === 'number' && Object.is(value, -0)) return '-0';
 return JSON.stringify(value, (_, v) => typeof v === 'number' && !Number.isFinite(v) ? String(v) : v, 2);
}
function renderCards(value, host) {
 host.replaceChildren();
 if (Array.isArray(value)) {
  if (!value.length) {const p=document.createElement('p');p.className='empty';p.textContent='No items returned';host.append(p);return;}
  const grid=document.createElement('div');grid.className='cards';
  value.slice(0,40).forEach((item,index)=>{const card=document.createElement('div');card.className='card';const label=document.createElement('small');label.textContent='Item '+(index+1);card.append(label);appendValue(item,card);grid.append(card);});host.append(grid);
  if(value.length>40){const p=document.createElement('p');p.textContent='Showing 40 of '+value.length+' items.';host.append(p);}
 } else if(value && typeof value==='object'){const card=document.createElement('div');card.className='card';appendValue(value,card);host.append(card);}
 else {const p=document.createElement('div');p.className='hero';p.textContent=typeof value==='boolean'?(value?'Yes · true':'No · false'):value===undefined?'No value · undefined':value===''?'Empty string':String(value);host.append(p);}
}
function appendValue(value,host){
 if(value && typeof value==='object' && !Array.isArray(value))Object.entries(value).slice(0,40).forEach(([key,item])=>{const row=document.createElement('div');row.className='field';const label=document.createElement('small');label.textContent=key;const text=document.createElement('span');text.className='value';text.textContent=typeof item==='string'?item:readable(item);row.append(label,text);host.append(row);});
 else {const text=document.createElement('div');text.className='value';text.textContent=typeof value==='string'?value:readable(value);host.append(text);}
}
function update(){
 try{
  if(input.value.length>10000)throw new Error('Keep this sample below 10,000 characters.');
  const data=JSON.parse(input.value);const before=readable(data);const result=transform(data);const after=readable(data);
  renderCards(result,output);source.replaceChildren();(Array.isArray(data)?data.slice(0,40):[data]).forEach(item=>{const card=document.createElement('div');card.className='source-card';card.textContent=typeof item==='string'?item:readable(item);source.append(card);});document.getElementById('source-status').textContent=before===after?'Data values unchanged':'Data values changed in place';document.getElementById('inspect').textContent='Input before:\n'+before+'\n\nReturned result:\n'+readable(result)+'\n\nSource after:\n'+after;
  output.classList.remove('error');status.textContent='Preview updated. '+(before===after?'Data values unchanged.':'Source data changed in place.');
 }catch(error){output.replaceChildren();output.classList.add('error');output.textContent=error.name+': '+error.message;source.replaceChildren();document.getElementById('source-status').textContent='';document.getElementById('inspect').textContent='';status.textContent='Check the JSON and the input shape. Restore Original data to start again.';}
}
document.getElementById('run').addEventListener('click',update);
[['original',initial],['changed',alternate],['edge',empty]].forEach(([id,value])=>document.getElementById(id).addEventListener('click',()=>{input.value=value;update();}));

input.value=initial;update();
</script></body></html>
Check a shift against required skills: edit the sample and check the resulting value.

Follow one run

The required skills form the receiver Set. isSubsetOf checks that every required value belongs to the available Set and returns a boolean. Additional available skills do not affect a true result. Reversing the two Sets checks whether all available skills are required, which is a different question.

const data = {
  "required": [
    "first-aid",
    "check-in"
  ],
  "available": [
    "first-aid",
    "check-in",
    "translation"
  ]
};
const result = new Set(data.required).isSubsetOf(new Set(data.available));
console.log(result);

The starting sample returns:

true

The input is JSON-friendly, while the working collection is a Set. That distinction explains why the source panel can retain repeated values even when the result does not. When the result is a list, the wrapper converts the collection for display; a boolean or count is shown directly.

Check a shift against required skills: inspect the smaller example and compare its source with its result.
Check a shift against required skills: inspect the smaller example and compare its source with its result.

Check the case that changes the answer

An empty required Set is a subset of any Set. That is mathematically correct, but your scheduling form may still need validation to prevent an accidentally blank requirements list.

The result does not tell you which requirement is missing when coverage fails. Compute a directional difference if your interface needs to list uncovered skills. It also cannot tell you whether one person is covering conflicting responsibilities; these are simple skill values, not a complete staffing schedule with availability or workload.

The smaller version keeps the same expression with fewer visual elements. Use Edit code to inspect the complete HTML, or Copy code to run it as a local file. If the script does not execute, the JavaScript troubleshooting guide helps check placement and syntax.

Live exampletry it here, then copy the code
Share it as a link
<!doctype html><html lang="en"><head><meta charset="utf-8"><meta name="nos-demo-layout" content="responsive-lab"><meta name="viewport" content="width=device-width,initial-scale=1"><title>Check a shift against required skills</title><style>*{box-sizing:border-box}body{margin:0;background:#edf1f0;color:#172d36;font:15px/1.5 system-ui,sans-serif}main{max-width:1040px;margin:auto;padding:24px}header{display:flex;justify-content:space-between;gap:12px;align-items:center;margin-bottom:20px}.eyebrow{font-size:11px;font-weight:800;letter-spacing:2px;color:#55716b;text-transform:uppercase}h1{font-size:clamp(23px,4vw,34px);line-height:1.15;margin:8px 0}h2{font-size:16px;margin:0 0 12px}.tag{border:1px solid #bfceca;border-radius:99px;padding:6px 12px;font-size:12px;white-space:nowrap}.panel{background:white;border:1px solid #d4dfdb;border-radius:18px;padding:20px;margin-bottom:14px;box-shadow:0 9px 30px #1d393407}.grid{display:grid;grid-template-columns:1fr 1fr;gap:14px}.result{background:#17352f;color:#f7fffb}.result h2{color:#c9ded5}.cards{display:grid;gap:10px;grid-template-columns:repeat(auto-fit,minmax(135px,1fr))}.card{background:#f2ffc2;color:#17352f;padding:15px;border-radius:12px;min-width:0;overflow-wrap:anywhere}.card small{display:block;color:#536f60;font-size:11px;font-weight:700;text-transform:uppercase;margin-bottom:4px}.field{display:block;margin:8px 0}.field small{margin:0 0 3px;text-transform:none}.value{font-size:15px;font-weight:650;white-space:pre-wrap}.hero{font-size:clamp(28px,5vw,46px);font-weight:750;line-height:1.2;overflow-wrap:anywhere}.muted{color:#587169;font-size:13px}.result .muted{color:#bed7cb}pre{margin:0;white-space:pre-wrap;overflow-wrap:anywhere;font:13px/1.6 ui-monospace,monospace;max-height:220px;overflow:auto}.toolbar{display:flex;flex-wrap:wrap;gap:8px;margin:12px 0}button{background:#17352f;color:white;border:0;border-radius:10px;padding:10px 14px;font:600 13px system-ui;cursor:pointer}button.secondary{background:#edf3ea;color:#244337;border:1px solid #d0ddd0}button:hover{filter:brightness(.94)}button:focus-visible,textarea:focus-visible,summary:focus-visible{outline:3px solid #749919;outline-offset:3px}textarea{display:block;width:100%;height:126px;border:1px solid #bccfc5;border-radius:10px;background:#fafcf9;padding:12px;font:13px/1.5 ui-monospace,monospace;resize:vertical;color:#15362c}label{font-weight:650;font-size:13px}details{margin-top:16px}summary{cursor:pointer;font-size:13px;font-weight:650}#status{min-height:20px;font-size:12px;color:#526f60;margin-top:8px}.error{color:#ffcfbf}.footer{font-size:12px;color:#627b72;margin-top:16px}.empty{font-size:18px;color:#c7dacd}.source-card{background:#f4f7f3;border:1px solid #dbe4d8;border-radius:10px;padding:12px;overflow-wrap:anywhere;white-space:pre-wrap}.source-list{display:flex;flex-wrap:wrap;gap:8px;max-height:170px;overflow:auto}.source-list .source-card{flex:1;min-width:100px}.code{background:#f5f8f2}mark{background:#f2ffc2;color:#183a2d;padding:2px 5px;border-radius:4px}@media(max-width:620px){main{padding:15px}.grid{grid-template-columns:1fr}.panel{padding:16px}header{align-items:flex-start}.tag{display:none}h1{font-size:25px}.cards{grid-template-columns:repeat(auto-fit,minmax(125px,1fr))}.source-list{max-height:120px}pre{max-height:150px}}</style></head><body><main>
<header><div><div class="eyebrow">JavaScript • Set isSubsetOf</div><h1>Check a shift against required skills</h1><div class="muted">One input, one operation, one observable result.</div></div><span class="tag">Starter example</span></header>
<section class="panel"><label for="input">Input data (JSON)</label><textarea id="input" maxlength="10000" spellcheck="false"></textarea><div class="toolbar"><button id="run" type="button">Update preview</button><button class="secondary" id="original" type="button">Original data</button><button class="secondary" id="changed" type="button">Changed data</button><button class="secondary" id="edge" type="button">Empty / zero case</button></div><div id="status" role="status" aria-live="polite"></div></section>
<div class="grid"><section class="panel code"><h2>The operation</h2><pre id="expression"></pre></section><section class="panel result"><h2>Returned result</h2><pre id="result" aria-live="polite"></pre></section></div><section class="panel"><h2>Source after the operation</h2><pre id="source"></pre></section>
<p class="footer">Local sample data only. No server request, account, or installation is needed.</p></main><script>
'use strict';
const initial = "{\"required\":[\"first-aid\",\"check-in\"],\"available\":[\"first-aid\",\"check-in\",\"translation\"]}";
const alternate = "{\n  \"required\": [\n    \"first-aid\"\n  ],\n  \"available\": [\n    \"first-aid\",\n    \"check-in\"\n  ]\n}";
const empty = "{}";
const input = document.getElementById('input');
const output = document.getElementById('result');
const source = document.getElementById('source');
const status = document.getElementById('status');
function transform(data) { return new Set(data.required).isSubsetOf(new Set(data.available)); }
function readable(value) {
 if (value === undefined) return 'undefined';
 if (typeof value === 'number' && Object.is(value, -0)) return '-0';
 return JSON.stringify(value, (_, v) => typeof v === 'number' && !Number.isFinite(v) ? String(v) : v, 2);
}
function renderCards(value, host) {
 host.replaceChildren();
 if (Array.isArray(value)) {
  if (!value.length) {const p=document.createElement('p');p.className='empty';p.textContent='No items returned';host.append(p);return;}
  const grid=document.createElement('div');grid.className='cards';
  value.slice(0,40).forEach((item,index)=>{const card=document.createElement('div');card.className='card';const label=document.createElement('small');label.textContent='Item '+(index+1);card.append(label);appendValue(item,card);grid.append(card);});host.append(grid);
  if(value.length>40){const p=document.createElement('p');p.textContent='Showing 40 of '+value.length+' items.';host.append(p);}
 } else if(value && typeof value==='object'){const card=document.createElement('div');card.className='card';appendValue(value,card);host.append(card);}
 else {const p=document.createElement('div');p.className='hero';p.textContent=typeof value==='boolean'?(value?'Yes · true':'No · false'):value===undefined?'No value · undefined':value===''?'Empty string':String(value);host.append(p);}
}
function appendValue(value,host){
 if(value && typeof value==='object' && !Array.isArray(value))Object.entries(value).slice(0,40).forEach(([key,item])=>{const row=document.createElement('div');row.className='field';const label=document.createElement('small');label.textContent=key;const text=document.createElement('span');text.className='value';text.textContent=typeof item==='string'?item:readable(item);row.append(label,text);host.append(row);});
 else {const text=document.createElement('div');text.className='value';text.textContent=typeof value==='string'?value:readable(value);host.append(text);}
}
function update(){
 try{
  if(input.value.length>10000)throw new Error('Keep this sample below 10,000 characters.');
  const data=JSON.parse(input.value);const before=readable(data);const result=transform(data);const after=readable(data);
  output.textContent=readable(result);source.textContent=after;
  output.classList.remove('error');status.textContent='Preview updated. '+(before===after?'Data values unchanged.':'Source data changed in place.');
 }catch(error){output.replaceChildren();output.classList.add('error');output.textContent=error.name+': '+error.message;source.replaceChildren();status.textContent='Check the JSON and the input shape. Restore Original data to start again.';}
}
document.getElementById('run').addEventListener('click',update);
[['original',initial],['changed',alternate],['edge',empty]].forEach(([id,value])=>document.getElementById(id).addEventListener('click',()=>{input.value=value;update();}));
document.getElementById('expression').textContent="new Set(data.required).isSubsetOf(new Set(data.available))";
input.value=initial;update();
</script></body></html>
Check a shift against required skills: complete HTML starter.

Make one edit you can predict

Remove first-aid from available and rerun. Then try an empty required list and decide whether the form should accept that case before checking coverage.

Remove check-in from available and expect false. Add it back and expect true. Keep translation in available throughout to prove extra skills do not matter. For a useful extension, show the required-minus-available difference beside the boolean so a failed coverage check gives the coordinator a concrete next action. Treat this as a coverage check rather than a full staffing decision. The available values do not describe skill proficiency, certification expiry, or overlapping shifts.

Check a shift against required skills: the changed preset supplies a second input to compare with the original.
Check a shift against required skills: the changed preset supplies a second input to compare with the original.

Keep strings double-quoted when editing this sample. A JSON parsing example explains the difference between a parse error and a valid value that the operation cannot use.

Before using the coverage flag in a schedule, ask a teammate to remove one required skill from the available list. Share the runnable HTML through NOS with those sample requirements, so both of you can check the same missing-skill case rather than interpret a bare boolean. Their preview edits are local to their run; sharing the HTML does not automatically synchronize those edits.

Source: MDN Set.isSubsetOf.

Questions people ask

What does Set.isSubsetOf do in this example?

The required skills form the receiver Set. isSubsetOf checks that every required value belongs to the available Set and returns a boolean. Additional available skills do not affect a true result. Reversing the two Sets checks whether all available skills are required, which is a different question.

What is the main mistake to avoid with Set.isSubsetOf?

An empty required Set is a subset of any Set. That is mathematically correct, but your scheduling form may still need validation to prevent an accidentally blank requirements list.

Can I run the example without installing anything?

Yes. Copy the complete HTML demo, save it as index.html, and open it in a browser. It uses only sample data and built-in JavaScript.

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