Set a Default Parameter Value For a JavaScript Function

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.

In ES6, you can simulate default named parameters via destructuring:

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

// sample call using an object
myFor({ start: 3, end: 0 });

// also OK
myFor();
myFor({});

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';
  ...
}

JavaScript function default parameters

Pass undefined as value

Check this: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters#passing_undefined_vs._other_falsy_values

const add = (a = 1, b = 1, c = 1) => a + b + c
console.log(add(4, undefined, 2))

javaScript function - why my default argument fails?

Default parameters is assigned only if no value or undefined is passed

let defaultStyle = {  one: 1, two: 2, three: 3 }

function styling(style = defaultStyle, ...ruleSetStock) {

  return ruleSetStock.map(ruleSet => {

    return style[ruleSet]

  })

}

console.log(styling(undefined, "one", "two", "three"))

Is having the first JavaScript parameter with default value possible?

You still could keep your add function the same by call it with params from destructed array like below snippet

function add(x = 5, y) {
  return x + y;
}

console.log(add(...[,3]));

How to use a default value for named parameter in Javascript

I think you're looking for default parameters

const someFunc = ({ a = "foo" }) => {

   console.log(a);

}

someFunc({}); // "foo"

someFunc({a: "bar"}); // "bar"

Override JavaScript default parameter with undefined

what would be an appropriate way to write foo to achieve this?

If you mean to default only when there is no parameter passed to the function call, then you need to check the arguments length, or to spread the arguments if you want to keep an arrow function.

const foo = (...args) => {

  const bar = args.length ? args[0] : "";

  console.log(bar)

}

foo(null) // null

foo(undefined) // undefined

foo(); // ""

How to pass an object as default argument to function in ecmascript?

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

called_method  = (output = {  key1: "value1" })  => {

    return output;

};

console.log("Default parameter value"); 

console.log(called_method()); 

const output2 = { key2: "value2" };

console.log("Paramter value passed in parameter"); 

console.log(called_method(output2)); 

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