Default parameters

In JavaScript, function parameters default to undefined. However, it's often useful to set a different default value. This is where default parameters can help.

In the following example, if no value is provided for b when multiply is called, b's value would be undefined when evaluating a * b and multiply would return NaN.

function multiply(a, b) {
  return a * b;
}

multiply(5, 2); // 10
multiply(5); // NaN !

In the past, the general strategy for setting defaults was to test parameter values in the function body and assign a value if they are undefined. In the following example, b is set to 1 if multiply is called with only one argument:

function multiply(a, b) {
  b = typeof b !== "undefined" ? b : 1;
  return a * b;
}

multiply(5, 2); // 10
multiply(5); // 5

With default parameters, checks in the function body are no longer necessary. Now, you can assign 1 as the default value for b in the function head:

function multiply(a, b = 1) {
  return a * b;
}

multiply(5, 2); // 10
multiply(5); // 5
multiply(5, undefined); // 5

Parameters are still set left-to-right, overwriting default parameters even if there are later parameters without defaults.

function f(x = 1, y) {
  return [x, y];
}

f(); // [1, undefined]
f(2); // [2, undefined]

The default parameter initializers live in their own scope, which is a parent of the scope created for the function body.

This means that earlier parameters can be referred to in the initializers of later parameters. However, functions and variables declared in the function body cannot be referred to from default value parameter initializers; attempting to do so throws a run-time ReferenceError. This also includes var-declared variables in the function body.

For example, the following function will throw a ReferenceError when invoked, because the default parameter value does not have access to the child scope of the function body:

function f(a = go()) {
  function go() {
    return ":P";
  }
}

f(); // ReferenceError: go is not defined

This function will print the value of the parameter a, because the variable var a is hoisted only to the top of the scope created for the function body, not the parent scope created for the parameter list, so its value is not visible to b.

function f(a, b = () => console.log(a)) {
  var a = 1;
  b();
}

f(); // undefined
f(5); // 5

The default parameter allows any expression, but you cannot use await or yield that would pause the evaluation of the default expression. The parameter must be initialized synchronously.

async function f(a = await Promise.resolve(1)) {
  return a;
}

In the second call in this example, even if the first argument is set explicitly to undefined (though not null or other falsy values), the value of the num argument is still the default.

function test(num = 1) {
  console.log(typeof num);
}

test(); // 'number' (num is set to 1)
test(undefined); // 'number' (num is set to 1 too)

// test with other falsy values:
test(""); // 'string' (num is set to '')
test(null); // 'object' (num is set to null)

The default argument is evaluated at call time. Unlike with Python (for example), a new object is created each time the function is called.

function append(value, array = []) {
  array.push(value);
  return array;
}

append(1); // [1]
append(2); // [2], not [1, 2]

This even applies to functions and variables:

function callSomething(thing = something()) {
  return thing;
}

let numberOfTimesCalled = 0;
function something() {
  numberOfTimesCalled += 1;
  return numberOfTimesCalled;
}

callSomething(); // 1
callSomething(); // 2

Parameters defined earlier (to the left) are available to later default parameters:

function greet(name, greeting, message = `${greeting} ${name}`) {
  return [name, greeting, message];
}

greet("David", "Hi"); // ["David", "Hi", "Hi David"]
greet("David", "Hi", "Happy Birthday!"); // ["David", "Hi", "Happy Birthday!"]

This functionality can be approximated like this, which demonstrates how many edge cases are handled:

function go() {
  return ":P";
}

function withDefaults(
  a,
  b = 5,
  c = b,
  d = go(),
  e = this,
  f = arguments,
  g = this.value,
) {
  return [a, b, c, d, e, f, g];
}

function withoutDefaults(a, b, c, d, e, f, g) {
  switch (arguments.length) {
    case 0:
    case 1:
      b = 5;
    case 2:
      c = b;
    case 3:
      d = go();
    case 4:
      e = this;
    case 5:
      f = arguments;
    case 6:
      g = this.value;
  }
  return [a, b, c, d, e, f, g];
}

withDefaults.call({ value: "=^_^=" });
// [undefined, 5, 5, ":P", {value:"=^_^="}, arguments, "=^_^="]

withoutDefaults.call({ value: "=^_^=" });
// [undefined, 5, 5, ":P", {value:"=^_^="}, arguments, "=^_^="]

You can use default value assignment with the destructuring assignment syntax.

A common way of doing that is to set an empty object/array as the default value for the destructured parameter; for example: [x = 1, y = 2] = []. This makes it possible to pass nothing to the function and still have those values prefilled:

function preFilledArray([x = 1, y = 2] = []) {
  return x + y;
}

preFilledArray(); // 3
preFilledArray([]); // 3
preFilledArray([2]); // 4
preFilledArray([2, 3]); // 5

// Works the same for objects:
function preFilledObject({ z = 3 } = {}) {
  return z;
}

preFilledObject(); // 3
preFilledObject({}); // 3
preFilledObject({ z: 2 }); // 2

• Functions guide
• Functions
• Rest parameters
• Nullish coalescing operator (??)