जब जावास्क्रिप्ट में वेर, लेट और कास्ट का उपयोग करना है [टायलर मैकगिनिस लेख से अनुवाद]

क्षेत्र

"यदि एक फ़ंक्शन घोषणा के अंदर एक चर घोषणा होती है, तो चर को इस फ़ंक्शन के स्थानीय दायरे में परिभाषित किया जाता है ..."

 function getDate () { var date = new Date() return date } getDate() console.log(date) // NOT OK: Reference Error 

 function getDate () { var date = new Date() function formatDate () { return date.toDateString().slice(4) // OK } return formatDate() } getDate() console.log(date) // NOT OK: Reference Error 

 discountPrices([100, 200, 300], .5) 

 function discountPrices (prices, discount) { var discounted = [] for (var i = 0; i < prices.length; i++) { var discountedPrice = prices[i] * (1 - discount) var finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } return discounted } 

 function discountPrices (prices, discount) { var discounted = [] for (var i = 0; i < prices.length; i++) { var discountedPrice = prices[i] * (1 - discount) var finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } 

"रेजिंग"

 function discountPrices (prices, discount) { var discounted = undefined var i = undefined var discountedPrice = undefined var finalPrice = undefined discounted = [] for (var i = 0; i < prices.length; i++) { discountedPrice = prices[i] * (1 - discount) finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } 

 function discountPrices (prices, discount) { console.log(discounted) // undefined var discounted = [] for (var i = 0; i < prices.length; i++) { var discountedPrice = prices[i] * (1 - discount) var finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } 

var, जाने या कास्ट

 function discountPrices (prices, discount) { var discounted = [] for (var i = 0; i < prices.length; i++) { var discountedPrice = prices[i] * (1 - discount) var finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } 

 function discountPrices (prices, discount) { let discounted = [] for (let i = 0; i < prices.length; i++) { let discountedPrice = prices[i] * (1 - discount) let finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } discountPrices([100, 200, 300], .5) // NOT OK: ReferenceError: i is not defined 

 var VS let var:     let:     

 function discountPrices (prices, discount) { console.log(discounted) // undefined var discounted = [] for (var i = 0; i < prices.length; i++) { var discountedPrice = prices[i] * (1 - discount) var finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } 

 function discountPrices (prices, discount) { console.log(discounted) // NOT OK: ReferenceError let discounted = [] for (let i = 0; i < prices.length; i++) { let discountedPrice = prices[i] * (1 - discount) let finalPrice = Math.round(discountedPrice * 100) / 100 discounted.push(finalPrice) } console.log(i) // 3 console.log(discountedPrice) // 150 console.log(finalPrice) // 150 return discounted } 

 var VS let var:        undefined         . let:       ReferenceError        . 

चलो या कांस्टेबल

 let name = 'Tyler' const handle = 'tylermcginnis' name = 'Tyler McGinnis' // OK handle = '@tylermcginnis' // NOT OK: TypeError: Assignment to constant variable. 

 const person = { name: 'Kim Kardashian' } person.name = 'Kim Kardashian West' // OK person = {} // NOT OK: Assignment to constant variable. 

 var VS let VS const var:        undefined         . let:       ReferenceError        . const:       ReferenceError        .