JavaScript

The typeof operator in JavaScript is used to check the data type of a variable or value. It returns a string indicating the type.

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✅ Syntax:

typeof operand

OR

typeof(operand)

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🔢 Common Examples

typeof "Hello"     // "string"
typeof 42          // "number"
typeof true        // "boolean"
typeof undefined   // "undefined"
typeof { name: "Ali" } // "object"
typeof [1, 2, 3]   // "object"
typeof null        // "object" ❗ (known quirk)
typeof function() {} // "function"

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📘 typeof Results Table

Value	typeof Result
"Hello"	"string"
123	"number"
true / false	"boolean"
undefined	"undefined"
{} (object literal)	"object"
[] (array)	"object"
null	"object" ❗
function() {}	"function"
Symbol("id")	"symbol"
BigInt(1234567890123456789)	"bigint"

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⚠️ Important Notes

1. ❗ typeof null returns "object"

This is a known bug in JavaScript that exists for backward compatibility.

typeof null; // "object" (even though it's null)

You can check for null more reliably using:

value === null

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2. Arrays return "object"

Even though arrays are different from plain objects:

typeof [1, 2, 3]; // "object"

To specifically check for arrays, use:

Array.isArray([1, 2, 3]); // true

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🔎 Checking Multiple Types

You can use typeof inside conditionals:

function checkType(val) {
  if (typeof val === "string") {
    console.log("This is a string.");
  } else if (typeof val === "number") {
    console.log("This is a number.");
  } else {
    console.log("Unknown type.");
  }
}

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🧪 Real-world Example

const inputs = [42, "hello", true, null, undefined, [1, 2], {}, () => {}];

inputs.forEach(item => {
  console.log(`${item} => ${typeof item}`);
});

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✅ Summary

• typeof returns a string indicating the type of a value.
• Useful for debugging and type checking.
• Be aware of quirks like:
  • typeof null → "object"
  • typeof array → "object"

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