typeof Keyword
This page was last reviewed on Dec 13, 2023.
Dot Net Perls
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.
Here We print the result of typeof to the page with console.log. We see the lowercase type strings.
Info We use an if, else chain to test the result of typeof. In this way we can test the types of array elements.
// 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"); }
Summary. 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.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Dec 13, 2023 (edit).
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