Set a default parameter value for a JavaScript function

From ES6/ES2015, default parameters are in the language specification.

function read_file(file, delete_after = false) {  // Code}

just works.

Reference: Default Parameters - MDN

Default function parameters allow formal parameters to be initialized with default values if no value or undefined is passed.

You can also simulate default named parameters via destructuring:

// the `= {}` below lets you call the function without any parametersfunction myFor({ start = 5, end = 1, step = -1 } = {}) { // (A)    // Use the variables `start`, `end` and `step` here    ···}

Pre ES2015,

There are a lot of ways, but this is my preferred method — it lets you pass in anything you want, including false or null. (typeof null == "object")

function foo(a, b) {  a = typeof a !== 'undefined' ? a : 42;  b = typeof b !== 'undefined' ? b : 'default_b';  ...}

function read_file(file, delete_after) {    delete_after = delete_after || "my default here";    //rest of code}

This assigns to delete_after the value of delete_after if it is not a falsey value otherwise it assigns the string "my default here". For more detail, check out Doug Crockford's survey of the language and check out the section on Operators.

This approach does not work if you want to pass in a falsey value i.e. false, null, undefined, 0 or "". If you require falsey values to be passed in you would need to use the method in Tom Ritter's answer.

When dealing with a number of parameters to a function, it is often useful to allow the consumer to pass the parameter arguments in an object and then merge these values with an object that contains the default values for the function

function read_file(values) {    values = merge({         delete_after : "my default here"    }, values || {});    // rest of code}// simple implementation based on $.extend() from jQueryfunction merge() {    var obj, name, copy,        target = arguments[0] || {},        i = 1,        length = arguments.length;    for (; i < length; i++) {        if ((obj = arguments[i]) != null) {            for (name in obj) {                copy = obj[name];                if (target === copy) {                    continue;                }                else if (copy !== undefined) {                    target[name] = copy;                }            }        }    }    return target;};

to use

// will use the default delete_after valueread_file({ file: "my file" }); // will override default delete_after valueread_file({ file: "my file", delete_after: "my value" }); 

I find something simple like this to be much more concise and readable personally.

function pick(arg, def) {   return (typeof arg == 'undefined' ? def : arg);}function myFunc(x) {  x = pick(x, 'my default');}