Checks if value is undefined. Part of Lodash Lang (stable since 0.1.0 — not deprecated). Note: same as value === undefined — null returns false. Use _.isNil when both should match.
Pass the value you want to inspect or convert. Read the return type carefully before you use the result.
Call _.isUndefined. Start with this line.
_.isUndefined(value)| Call | What it does |
|---|---|
_.isUndefined(value) | Primary signature. |
Prefer import isUndefined from "lodash/isUndefined" when you want a smaller bundle.
_.isUndefined(value)| Parameter | Description |
|---|---|
| value | Required. Value to check |
| Return Value | Description |
|---|---|
| * | true if value is undefined, else false |
void 0 vs nullOnly the undefined primitive returns true.
console.log(_.isUndefined(void 0)); // true
console.log(_.isUndefined(null)); // falseundefined → true.false.null._.isNil.isUndefined vs isNilSee how the two helpers disagree on null.
console.log(_.isUndefined(undefined)); // true
console.log(_.isNil(undefined)); // true
console.log(_.isUndefined(null)); // false
console.log(_.isNil(null)); // trueisUndefined and isNil agree → true.isUndefined rejects; isNil accepts.isUndefined; any missing → isNil.value == null behaves like isNil, not isUndefined.undefinedKeep 0, empty string, and false as present values.
console.log(_.isUndefined(0)); // false
console.log(_.isUndefined("")); // false
console.log(_.isUndefined(false)); // false
console.log(_.isUndefined(NaN)); // falsefalse.=== undefined succeeds.!value when you mean “is undefined.”Absent object keys and omitted parameters are undefined.
var user = { name: "Ada" };
console.log(_.isUndefined(user.age)); // true
function greet(name) {
return _.isUndefined(name) ? "Guest" : name;
}
console.log(greet()); // Guest
console.log(greet("Ada")); // Adauser.age was never set → undefined.greet() leaves name undefined.ReferenceError before Lodash runs.nullModel APIs that leave a field unset vs clear it with null.
function statusOf(nickname) {
if (_.isUndefined(nickname)) {
return "not set";
}
if (_.isNull(nickname)) {
return "cleared";
}
return "set: " + nickname;
}
console.log(statusOf(undefined)); // not set
console.log(statusOf(null)); // cleared
console.log(statusOf("Ada")); // set: Ada"not set"."cleared".isNil would lose that distinction._.isUndefined has shipped since 0.1.0 and is available in Lodash 4.x (and earlier 3.x builds). It runs anywhere Lodash runs — modern browsers (via CDN or bundle) and Node.js / Bun / Deno with npm install lodash. Native engines express the same idea with value === undefined, but not this helper by name.
Available in [email protected] full builds and the lodash/isUndefined modular import. Works in evergreen browsers and current Node LTS when Lodash is installed.
Bottom line: Safe for production once Lodash is on the page or in your bundle. Prefer import isUndefined from "lodash/isUndefined" for smaller builds. Equivalent to value === undefined as a named function.
Check whether a value is classified as a WeakMap.
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