functions.md

Functions

A set of defined functionality that can be called with or with parameters

• Functions
  • Using the function keywork
  • Anonymous functions
  • Arrow functions
  • Optional function values
  • Spread function ...
  • Generator function

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Using the function keywork

function add(num1, num2) {
    return num1 + num2;
}
console.log(add(1,2)); // returns 3

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Anonymous functions

The variable becomes the function.

const addUp = function (num1, num2) {
    return num1 + num2;  
}
console.log(addUp(4,5)); // prints 9

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Arrow functions

const result1 = (num1, num2) => {
    return num1 + num2;
    };
console.log(result1(3,4));

// or

const result2 = (num1, num2) => num1 + num2;
console.log(result2(1,2));

If you do not encapsulate the code after the => in {} then you do not have to say return, as this is implied. If you use {}, you must use return. ({} is needed if the code is multi-line.)

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Optional function values

function hello(firstName, lastName = 'you lamer!') {
    return "Hello " + firstName + " " + lastName;
}

console.log(hello("Matt", "P"));  // prints Hello Matt P
console.log(hello("Matt"));  // prints Hello Matt you lamer

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Spread function ...

Passes object as parameters to function

function spreadFunction(options = {}) {
    const config = { ...options };
    console.log(config); // prints object - { a: 'cat', b: 17 }
}

// call with object

spreadFunction({
    a: 'cat',
    b: 17
});

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Generator function

The function* declaration creates a binding of a new generator function to a given name. A generator function can be exited and later re-entered, with its context (variable bindings) saved across re-entrances. Fast and low memory infinite counter.

// generator function - retains context
function* infiniteCounter(prefix = 'id') {
    let cnt = 1;
    while (true) {
        yield `${prefix}-${cnt++}`;
    }
}

const userId = infiniteCounter('user');
const pageId = infiniteCounter('page');

console.log(userId.next().value);
console.log(userId.next().value);
console.log(pageId.next().value);
console.log(userId.next().value);
console.log(userId.next().value);

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