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JavaScript Rest Parameters and Spread Syntax

bello profile image Bello ・Updated on ・3 min read

In this article, we will see what the rest parameters and what spread syntax is all about.

The example below is a summary of this article.

const peopleGrades = (index, ...numbers) => {  /* ...rest */
  const array = [ ...numbers ]; // ...spread
  return array[index]; // 8 => [ 4, 5, 2, 8, 2 ][3]
};

peopleGrades(3, 4, 5, 2, 8, 2); // 8
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Rest Parameter

A function will only perform operations on parameters and variables in the function body.

See the example below:

const sum = (a, b) => {
  return a + b; // 2 + 3
}

console.log( sum(2, 3, 1, 10, 4) ); // 5
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Notice the operation in the function body is based on the number of parameters, not the number of arguments in the calling function.

We can re-consider passing all arguments including 1, 10, 4 to the parameter for a returned computation in the function body.

See the example below:

const sum = (a, b, c, d, e) => {
  return a + b + c + d + e; // 2 + 3 + 1 + 10 + 4
};

console.log( sum(2, 3, 1, 10, 4) ); // 20
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Passing arguments to the parameters shown above is not advisable. In JavaScript, it is advisable to use the Rest parameter to represent more than three parameters.

A rest parameter looks like ...args. The three dots ... represents more parameters.

See the syntax below:

func = (...args) => {
  ...
};

func(args);
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Where args = array of parameters name.

The example below proves that the args is a list of arguments.

const sumFunc = (...args) => {
  return args; 
};

// let's gather all arguments into the array 
console.log( sumFunc(2, 3, 1, 10, 4) ); // [ 2, 3, 1, 10, 4 ]
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Since args is an array of arguments, we can sum each argument in the array.

See the example below:

const sumFunc = (...args) => {
  const sum = args[0] + args[1] + args[2] + args[3] + args[4];
  return sum; // 2 + 3 + 1 + 10 + 4
};

// let's sum all arguments in the array 
console.log( sumFunc(2, 3, 1, 10, 4) ); // 20
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See another example below:

const sumFunc = (sum = 0, ...args) => {

  for (let arg of args) {
    sum = sum + arg ;
  }

  return sum;
};

console.log( sumFunc(2, 3, 1, 10, 4) ); // 20
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Let's see another example below:

const employee = (name, ...titles) => {
  const personTitles = `${name} is a ${titles[0]}, 
  ${titles[1]}, ${titles[2]}, and ${titles[3]}`;
  return personTitles;
};

// let's gather all arguments into array 
console.log(
  employee('John', 'web developer', 'doctor',
    'consultant', 'professor')
);
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The ...args must always be the last parameter.

The example below will be invalid.

const employee= (...titles, name) => {
  const personTitles = `${name} is a ${titles[0]}, 
  ${titles[1]}, ${titles[2]}, and ${titles[3]}`;
  return personTitles;
};

console.log( 
  employee('web developer', 'doctor', 
  'consultant', 'professor', 'John')
  ); // SyntaxError: Rest parameter must be last formal parameter
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Spread Syntax

The spread works on iterable like strings, arrays, objects, etc.

The spread syntax is quite different from the rest syntax.

See the example below:

const staff = (str) => {
  const employee = [ ...str ];
  return employee;
};

console.log( staff('Bello') ); // [ 'B', 'e', 'l', 'l', 'o' ]
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See another example below:

const staff = (recruiter) => {
  const batch1Recruits = [ 'John', 'Sarah', 'Angelina', 'Tom' ];
  const batch2Recruits = [ 'Bob', 'Jack', 'Lucky', 'Ali' ];
  const newEmployees = [ recruiter, ...batch1Recruits, ...batch2Recruits ];
  return newEmployees;
};

console.log( staff('Patricia') );
/*
[
  'Patricia', 'John',
  'Sarah',    'Angelina',
  'Tom',      'Bob',
  'Jack',     'Lucky',
  'Ali'
]
*/
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The spread syntax works for iterable - string, arrays, objects, maps, sets, etc

Let see one last example, but on an object:

const myDetails = { age: 27, isDoctor: false };

const dataBase = { name: 'Bello', ...myDetails };

console.log(dataBase);
// { name: 'Bello', age: 27, isDoctor: false }
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Happy coding!!!

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