Typeof

With this Node.js operator, we get a string that indicates the type of a variable. A variable or array element can be passed to typeof.

For performance, typeof is a light-weight operation. In benchmarks it appears to execute as fast as a null check in the Node.js runtime.

A first example

Here we have an array with three elements—a string, a number, and a function. In JavaScript functions too are objects.

// Create test array.
var values = ["cat", 100, function(){}];

// Write the result of typeof.
for (var i = 0; i < values.length; i++) {
    console.log("TYPEOF RESULT: " + typeof values[i]);
}

// Test the elements with typeof.
for (var i = 0; i < values.length; i++) {
    if (typeof values[i] === "string") {
        console.log("FOUND string");
    }
    else if (typeof values[i] === "number") {
        console.log("FOUND number");
    }
    else if (typeof values[i] === "function") {
        console.log("FOUND function");
    }
}
TYPEOF RESULT: string
TYPEOF RESULT: number
TYPEOF RESULT: function
FOUND string
FOUND number
FOUND function

Arrays

For arrays, we cannot use a "typeof array" expression. We must use the Array.isArray method. We can use isArray on literal arrays and arrays created with the Array constructor.

var test = [1, 2, 3];
// See if the value is an array.
if (Array.isArray(test)) {
    console.log("ISARRAY");
}
if (typeof test === "array") {
    // This does not work.
    console.log("ERROR");
}

var emptyArray = Array(100);
// We can also test an array made with the Array constructor.
if (Array.isArray(emptyArray)) {
    console.log("ISARRAY");
}
ISARRAY
ISARRAY

With the typeof operator we get a string that tells us an object's type. The typeof operator appears to perform fast in Node.js—about as fast as a null check.