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Javascript Arrow Function

#es6 #arrowfunction #javascript

Javascript Arrow is an alternative syntax to the old regular function.

There is two main use case of using a javascript arrow function. The first is that it makes the code shorter and concise like below.

No argument 
// Regular Function
function sayHello() {
  return 'Hello'
}

// Arrow Function
const sayHello = () => 'Hello'
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One argument 
// Regular Function
function addTen(number) {
  return number + 10
}

// Arrow Function
// Notice that we can remove the parenthesis around the parameter when argument is just one. It is optional.
const addTen = num => number + 10
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Two or more arguments with multiple function body statements. We have to keep the curly braces and the return keyword for multiple function body statements. 
// Regular Function
function addTen(number1, number2) {
  const ten = 10                   // function body line #1
  return number1 + number2 + 10    // function body line #2
}

// Arrow Function
const addTen = (number1, number2) => {
  const ten = 10
  return number1 + number2 + 10
}
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The second use case of the arrow function is to handle the scope better. Let see a very simple example.

const person = {
    myName: ‘John Doe’,
    sayName () {
        setTimeout(function() {
             console.log(`Hi, ${this.myName}`)
        }, 1000)
    }

}
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When we invoke

person.sayName()
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We expect to see

Hi, John Doe
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Unfortunately, will get below.
It is because the function is pointing to the window object and window object doesn't have a method called sayName.

Hi, undefined
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To fix the context issue, engineers use these workarounds

Fix #1: 

// Save the context and use that context
const person = {
    myName: ‘John Doe’,
    sayName () {
        const _this = this
        setTimeout(function() {
             console.log(`Hi, ${_this.myName}`)
        }, 1000)
    }
 
}
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Fix #2: 

// Use bind to ensure that context is set and kept
const person = {
    myName: ‘John Doe’,
    sayName () {
        setTimeout(function() {
             console.log(`Hi, ${this.myName}`)
        }.bind(this), 1000)
    }
 
}
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Fix #3 - Modern solution

Arrow function behaves like a variable, it establishes the value of this (context) where the arrow function is created

// Using Arrow Function. 
const person = {
    myName: ‘John Doe’,
    sayName () {
        setTimeout(() => console.log(`Hi, ${this.myName}`), 1000)
    }
 
}
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peerreynders • • Edited 
// function declaration
function sayHello() {
  return 'Hello'
}

// function expression
const sayHello2 = function() {
  return 'Hello'
};

// arrow function expression
const sayHello3 = () => 'Hello';
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Function declarations are hoisted while function expressions are not.

// Using a closure
function makePerson(myName) {
  const greet = () => console.log(`Hi, I'm ${myName}`);
  return {
    myName,
    sayName() {
      setTimeout(greet, 1000);
    },
  };
}

const person = makePerson('John Doe');
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// constructor function
function Person(myName) {
  const greeting = `Hi, I'm ${myName}`;
  this.greet = () => console.log(greeting);
  this.myName = name;
}

Person.prototype.sayName = function () {
  setTimeout(this.greet, 1000);
};

const person = new Person('John Doe');
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