Default parameters

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Default parameters allow you to set default values for function parameters if the function is called without arguments, or if the parameter is explicitly passed the value undefined.

Teacher

In JavaScript, function parameters⚙️ that are not passed values when called are defaulted to undefined. However, in some cases it may be useful to set a different default value. This is where the default options are intended.

Syntax

book

Live Editor

function learnJavaScript() {
  const multiply = (a, b = 1) => {
    // The default value b is 1
    return a * b
  }
  // If b is undefined, then it will be assigned a default value
  return multiply(5, 2) // remove the comma, second argument and get 5 * 1
}

function learnJavaScript() {
  const multiply = (a, b = 1) => {
    // The default value b is 1
    return a * b
  }
  // If b is undefined, then it will be assigned a default value
  return multiply(5, 2) // remove the comma, second argument and get 5 * 1
}

Result

Passing other "false" values

basketball

If any value other than undefined is passed to the formal parameter during the call, including one of the" false "values such as false , 0, " ", '', null,NaN, then in this case the default value will not be assigned to the parameter. In this case, you need to write the code yourself that will catch these "false values".

Examples of

Math

In the default parameters, you can use the values of the previous (located to the left in the list) parameters:

Live Editor

function learnJavaScript() {
  const greet = (name, greeting, message = greeting + ' ' + name) => {
    return [name, greeting, message]
  }

  return greet('David ', 'Hi ')
}

function learnJavaScript() {
  const greet = (name, greeting, message = greeting + ' ' + name) => {
    return [name, greeting, message]
  }

  return greet('David ', 'Hi ')
}

Result

An example of a function with and without default parameters :

Live Editor

function learnJavaScript() {
  const withDefaults = (a = 1, b = 3, c = 2) => {
    // Function with default parameters
    return [a, b, c]
  }
  const withoutDefaults = (a, b, c) => {
    // Function without default parameters
    if (a == undefined) {
      a = 1
    }
    if (b == undefined) {
      b = 3
    }
    if (c == undefined) {
      c = 2
    }
    return [a, b, c]
  }

  return withDefaults()
}

function learnJavaScript() {
  const withDefaults = (a = 1, b = 3, c = 2) => {
    // Function with default parameters
    return [a, b, c]
  }
  const withoutDefaults = (a, b, c) => {
    // Function without default parameters
    if (a == undefined) {
      a = 1
    }
    if (b == undefined) {
      b = 3
    }
    if (c == undefined) {
      c = 2
    }
    return [a, b, c]
  }

  return withDefaults()
}