Set.has in JavaScript: check whether a guest is on the list

Explore Set.has through this working example: check whether a guest is on the list. Change the input and inspect the result before using your own data.

A check-in screen needs a simple answer: is Owen on the invited guest list? Counting matches or rebuilding the whole list would obscure that decision. This example asks the collection one direct membership question and displays the resulting boolean.

The starting result is true because Owen appears in the collection. It is not the number of matching invitations, and it is not permission to admit someone automatically. It simply answers the membership question represented by these strings. A real check-in screen can use that answer to choose the next prompt or show an appropriate status.

Check whether a guest is on the list: source values beside the original result.
Check whether a guest is on the list: 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 whether a guest is on the list</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 has</div><h1>Check whether a guest is on the list</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 = "{\"guests\":[\"Maya\",\"Owen\",\"Nina\"],\"name\":\"Owen\"}";
const alternate = "{\n  \"guests\": [\n    \"Maya\",\n    \"Owen\"\n  ],\n  \"name\": \"Owen\"\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.guests).has(data.name); }
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 whether a guest is on the list: edit the sample and check the resulting value.

Follow one run

A fresh Set contains the guest names, and has checks the requested string. Owen produces true; a name absent from the collection produces false. The result is a boolean rather than a position or the matching name, and the source array stays unchanged.

const data = {
  "guests": [
    "Maya",
    "Owen",
    "Nina"
  ],
  "name": "Owen"
};
const result = new Set(data.guests).has(data.name);
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 whether a guest is on the list: inspect the smaller example and compare its source with its result.
Check whether a guest is on the list: inspect the smaller example and compare its source with its result.

Check the case that changes the answer

A false result does not explain why a lookup failed. Extra spaces or different capitalization can cause a mismatch, so normalize both sides when your product permits it.

Calling has on an empty Set returns false for Owen. Asking about an empty string can return true if that exact empty string was stored, which is why validating a guest name is separate from searching for it. Do not use the result as evidence that the source data itself was complete or trustworthy.

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 whether a guest is on the list</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 has</div><h1>Check whether a guest is on the list</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 = "{\"guests\":[\"Maya\",\"Owen\",\"Nina\"],\"name\":\"Owen\"}";
const alternate = "{\n  \"guests\": [\n    \"Maya\",\n    \"Owen\"\n  ],\n  \"name\": \"Owen\"\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.guests).has(data.name); }
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.guests).has(data.name)";
input.value=initial;update();
</script></body></html>
Check whether a guest is on the list: complete HTML starter.

Make one edit you can predict

Try Owen, owen, and a missing guest. Decide whether your own check-in screen should treat those first two spellings as the same person.

Try a requested name of Maya and expect true, then Leo and expect false. Add Leo to the guests array and his result becomes true. The useful exercise is making one membership change at a time, so you can distinguish a missing guest from a spelling mismatch in an otherwise present guest. Before connecting the lookup to admission rules, write down the exact identifier being searched. A display name, an email address, and a ticket ID are different inputs.

Check whether a guest is on the list: the changed preset supplies a second input to compare with the original.
Check whether a guest is on the list: 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 membership to choose a check-in message, ask another coordinator to try a known name and a missing one. Replace the sample in the HTML and share it through NOS. That keeps the exact search value and its boolean answer in the same example. Their preview edits are local to their run; sharing the HTML does not automatically synchronize those edits.

Source: MDN Set.has.

Questions people ask

What does Set.has do in this example?

A fresh Set contains the guest names, and has checks the requested string. Owen produces true; a name absent from the collection produces false. The result is a boolean rather than a position or the matching name, and the source array stays unchanged.

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

A false result does not explain why a lookup failed. Extra spaces or different capitalization can cause a mismatch, so normalize both sides when your product permits it.

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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