For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt, and this page is available as Markdown at /reference/predicate/isUndefined.md.
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    isUndefined es-toolkit

    Checks if a given value is undefined.

    const result = isUndefined(value);

    Usage

    isUndefined(value)

    Use isUndefined when you want to check if a value is undefined. It's useful for checking whether a variable is initialized or whether an optional property exists.

    import { isUndefined } from 'es-toolkit/predicate';

// undefined values
console.log(isUndefined(undefined)); // true
console.log(isUndefined(void 0)); // true

let uninitialized: string;
console.log(isUndefined(uninitialized)); // true

// Non-undefined values
console.log(isUndefined(null)); // false
console.log(isUndefined('')); // false
console.log(isUndefined(0)); // false
console.log(isUndefined(false)); // false
console.log(isUndefined({})); // false
console.log(isUndefined([])); // false

    Parameters

    valueunknownrequired

    The value to check if it's undefined.

    Returns

    Returns true if the value is undefined, false otherwise.

    value is undefined
    Source: es-toolkit

    Re-exported verbatim from es-toolkit. Implementation, edge cases, and performance behavior are owned upstream. This page mirrors the documentation at the pinned version; the linked source is authoritative.