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TypeError: "x" is (not) "y"

Message

    TypeError: "x" is (not) "y"

    Examples:
    TypeError: "x" is undefined
    TypeError: "x" is null
    TypeError: "undefined" is not an object
    TypeError: "x" is not an object or null
    TypeError: "x" is not a symbol

In this article

Error type

TypeError.

What went wrong?

. undefined null .

, Object.create() Symbol.keyFor() .

Examples

Invalid cases

js
// undefined and null cases on which the substring method won't work
var foo = undefined;
foo.substring(1); // TypeError: foo is undefined

var foo = null;
foo.substring(1); // TypeError: foo is null

// Certain methods might require a specific type
var foo = {};
Symbol.keyFor(foo); // TypeError: foo is not a symbol

var foo = "bar";
Object.create(foo); // TypeError: "foo" is not an object or null

Fixing the issue

undefined null null typeof .

js
if (typeof foo !== "undefined") {
  // Now we know that foo is defined, we are good to go.
}

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