How to Check if a Function returns True in JavaScript

Check if a Function returns True in JavaScript #

To check if a function returns true:

1. Call the function and check if its return value is equal to true.
2. If the condition is met, the function returns true.

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function doWork() {
  // logic ...
  return true;
}

// 👇️ Check if returns explicitly `true`
if (doWork() === true) {
  // 👇️ this runs
  console.log('✅ function returns true');
} else {
  console.log('the function does NOT return true');
}

If the condition is met, the if block is run, otherwise, the else block runs.

Alternatively, you can check if a function returns a truthy value.

Check if a Function returns a Truthy value in JavaScript #

To check if a function returns a truthy value:

1. Call the function and use its return value in an if statement.
2. The if block is only run if the function returns a truthy value.

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function doWork() {
  // logic ...
  return true;
}

// 👇️ Check if returns Truthy value
if (doWork()) {
  // 👇️ this runs
  console.log('✅ function returns TRUTHY value');
} else {
  console.log('The function does NOT return a truthy value');
}

The falsy values in JavaScript are: false, null, undefined, 0, "" (empty string), NaN (not a number).

All other values are truthy.

The if statement checks if the return value of the function is truthy.

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function doWork() {
  // logic ...
  return 'bobbyhadz.com';
}

// 👇️ Check if returns Truthy value
if (doWork()) {
  console.log('✅ function returns TRUTHY value');
} else {
  console.log('The function does NOT return a truthy value');
}

The function in the example returns a string and all non-empty strings are truthy values, so the if block runs.

If you pass a value to the Boolean() constructor and it returns true, then the if condition will be met and the if block will run.

Here are some examples of passing truthy and falsy values to the Boolean() constructor.

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// 👇️ truthy values
console.log(Boolean('hello')); // 👉️ true
console.log(Boolean(1)); // 👉️ true
console.log(Boolean([])); // 👉️ true
console.log(Boolean({})); // 👉️ true

// 👇️ falsy values
console.log(Boolean('')); // 👉️ false
console.log(Boolean(0)); // 👉️ false
console.log(Boolean(undefined)); // 👉️ false
console.log(Boolean(null)); // 👉️ false

Check if a function returns a truthy value using logical AND (&&) #

You can also use the logical AND (&&) operator to check if a function returns a truthy value.

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function doWork() {
  // logic ...
  return true;
}

function sum(a, b) {
  return a + b;
}

const result = doWork() && sum(10, 15);
console.log(result); // 👉️ 25

The logical AND (&&) operator returns the value to its left if it's falsy, otherwise, the value to its right is returned.

The doWork() function returns a truthy value, so the sum() function is invoked and its value is returned.

You can also explicitly check if the value is equal to true even though this is less common.

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function doWork() {
  return 'bobbyhadz.com';
}

function sum(a, b) {
  return a + b;
}

const result = doWork() === true && sum(10, 15);
console.log(result); // 👉️ false

The function returns a truthy value, but it doesn't return true, so the logical AND (&&) operator short-circuits returning the value to the left.

Here is an example of using the operator with a function that returns a falsy value.

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function doWork() {
  // logic ...
  return false;
}

function sum(a, b) {
  return a + b;
}

const result = doWork() && sum(10, 15);
console.log(result); // 👉️ false

If the value to the left of the logical AND (&&) operator is falsy, the expression to the right is not evaluated at all.

Check if a function returns a truthy value using ternary operator #

You can also use the ternary operator to check if a function returns true or a truthy value.

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function doWork() {
  return true;
}

function sum(a, b) {
  return a + b;
}

const result = doWork() ? sum(10, 10) : 'abcd';
console.log(result); // 👉️ 20

The ternary operator is very similar to an if/else statement.

If the expression to the left of the question mark is truthy, the operator returns the value to the left of the colon, otherwise, the value to the right of the colon is returned.

You can also use this approach to check if the function returns true.

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function doWork() {
  return true;
}

function sum(a, b) {
  return a + b;
}

const result = doWork() === true ? sum(10, 10) : 'abcd';
console.log(result); // 👉️ 20

Here is an example where the function returns a falsy value.

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function doWork() {
  return '';
}

function sum(a, b) {
  return a + b;
}

const result = doWork() ? sum(10, 10) : 'abcd';
console.log(result); // 👉️ 'abcd'

The function returns a falsy value, so the expression after the colon is returned.

You can imagine that the value before the colon is the if block and the value after the colon is the else block.