Set.clear in JavaScript: reset a temporary selection

Explore Set.clear through this working example: reset a temporary selection. Change the input and inspect the result before using your own data.

A file picker lets someone select several attachments before sending a message. Its reset button should clear every temporary choice at once. This example makes that change visible as an empty list and a zero count while keeping the original input available.

The result contains count zero and an empty selected array. Both help explain what reset actually did: the working selection has no remaining filenames. The original list stays visible beside it so you can compare before and after. No filename disappeared from a disk, and no upload was started or canceled by this operation.

Reset a temporary selection: source values beside the original result.
Reset a temporary selection: 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>Reset a temporary selection</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 clear</div><h1>Reset a temporary selection</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 = "{\"selected\":[\"brief.pdf\",\"budget.xlsx\",\"mockup.png\"]}";
const alternate = "{\n  \"selected\": [\n    \"brief.pdf\",\n    \"budget.xlsx\"\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 (() => { const selected = new Set(data.selected); selected.clear(); return {count:selected.size, selected:Array.from(selected)}; })(); }
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>
Reset a temporary selection: edit the sample and check the resulting value.

Follow one run

The example constructs a working Set, then clear removes all its entries. The return value of clear is undefined, so the interface instead reads size and converts the remaining collection to an array. Clearing an already-empty Set also leaves it empty without an error.

const data = {
  "selected": [
    "brief.pdf",
    "budget.xlsx",
    "mockup.png"
  ]
};
const result = (() => { const selected = new Set(data.selected); selected.clear(); return {count:selected.size, selected:Array.from(selected)}; })();
console.log(result);

The starting sample returns:

{
  "count": 0,
  "selected": []
}

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.

Reset a temporary selection: inspect the smaller example and compare its source with its result.
Reset a temporary selection: inspect the smaller example and compare its source with its result.

Check the case that changes the answer

Clearing a Set only clears that collection. It does not delete files, cancel uploads, or modify the original JSON array used to construct the working selection.

Calling clear is different from assigning a new Set to a variable. clear empties the existing collection, so other code holding a reference to that same Set would see it empty as well. This small example creates a local Set, which keeps the effect contained within one preview run.

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
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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>Reset a temporary selection</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 clear</div><h1>Reset a temporary selection</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 = "{\"selected\":[\"brief.pdf\",\"budget.xlsx\",\"mockup.png\"]}";
const alternate = "{\n  \"selected\": [\n    \"brief.pdf\",\n    \"budget.xlsx\"\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 (() => { const selected = new Set(data.selected); selected.clear(); return {count:selected.size, selected:Array.from(selected)}; })(); }
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="(() => { const selected = new Set(data.selected); selected.clear(); return {count:selected.size, selected:Array.from(selected)}; })()";
input.value=initial;update();
</script></body></html>
Reset a temporary selection: complete HTML starter.

Make one edit you can predict

Replace the selected list with an empty array and rerun. Then add two filenames and confirm reset produces the same empty result in both cases.

Put only brief.pdf into selected and expect count zero and selected empty after running. Repeat with selected already empty and the output is identical. If you extend the example with a reset confirmation, base that message on the cleared collection rather than on clear returning a success boolean; it does not. If resetting needs an undo option, preserve the selection before clearing it. An empty result confirms reset but no longer contains the filenames needed to restore the choices.

Reset a temporary selection: the changed preset supplies a second input to compare with the original.
Reset a temporary selection: 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.

A reset button needs clear expectations about what disappears. Share this runnable HTML through NOS with a teammate after setting representative filenames, and let them verify the empty selection. The visible reset affects the sample collection; it never deletes the named files. Their preview edits are local to their run; sharing the HTML does not automatically synchronize those edits.

Source: MDN Set.clear.

Questions people ask

What does Set.clear do in this example?

The example constructs a working Set, then clear removes all its entries. The return value of clear is undefined, so the interface instead reads size and converts the remaining collection to an array. Clearing an already-empty Set also leaves it empty without an error.

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

Clearing a Set only clears that collection. It does not delete files, cancel uploads, or modify the original JSON array used to construct the working selection.

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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