var :
Before ES6, the var keyword was used to declare a variable in JavaScript. The var keyword has been around since the inception of JavaScript, and it’s what you will see in any pre ES6 code.
Variables declared using the var keyword are either globally or functionally scoped, they do not support block-level scope. This means that if a variable is defined in a loop or in an if statement it can be accessed outside the block and accidentally redefined leading to a buggy program. As a general rule, you should avoid using the var keyword.
Let : In many ways, the let keyword is very similar to the var keyword as they both allow you to reassign the value later on. The main difference between the two is that let deals with a block scope and although it can be reassigned it cannot be redeclared.
Const : Like the let keyword, the const keyword is also blocked scoped. However, declaring a variable with the const keyword means that it cannot only be redeclared but also it cannot be reassigned. Therefore, every const variable must be assigned a value at the time of declaration.
Variables declared using the var keyword are either globally or functionally scoped, they do not support block-level scope. This means that if a variable is defined in a loop or in an if statement it can be accessed outside the block and accidentally redefined leading to a buggy program. As a general rule, you should avoid using the var keyword.
Let : In many ways, the let keyword is very similar to the var keyword as they both allow you to reassign the value later on. The main difference between the two is that let deals with a block scope and although it can be reassigned it cannot be redeclared.
Const : Like the let keyword, the const keyword is also blocked scoped. However, declaring a variable with the const keyword means that it cannot only be redeclared but also it cannot be reassigned. Therefore, every const variable must be assigned a value at the time of declaration.
ForEach :
Foreach takes a callback function and run that callback function on each element of array one by one.Basically forEach works as a traditional for loop looping over the array and providing you array elements to do operations on them.
Filter : The main difference between forEach and filter is that forEach just loop over the array and executes the callback but filter executes the callback and check its return value. If the value is true element remains in the resulting array but if the return value is false the element will be removed for the resulting array.
Map : Map like filter & foreach takes a callback and run it against every element on the array but whats makes it unique is it generate a new array based on your existing array.
Map ran through every element of the array, multiplied it to 10 and returned the element which will be going to store inside our resulting array.
Like filter, map also returns an array. The provided callback to map modifies the array elements and save them into the new array upon completion that array get returned as the mapped array.
Find : Function .find() is also a search function like the previous but they differ in one small detail — this function returns only one match in an array. If in an array is more than one result, the function will return the first that has matched.
Filter : The main difference between forEach and filter is that forEach just loop over the array and executes the callback but filter executes the callback and check its return value. If the value is true element remains in the resulting array but if the return value is false the element will be removed for the resulting array.
Map : Map like filter & foreach takes a callback and run it against every element on the array but whats makes it unique is it generate a new array based on your existing array.
Map ran through every element of the array, multiplied it to 10 and returned the element which will be going to store inside our resulting array.
Like filter, map also returns an array. The provided callback to map modifies the array elements and save them into the new array upon completion that array get returned as the mapped array.
Find : Function .find() is also a search function like the previous but they differ in one small detail — this function returns only one match in an array. If in an array is more than one result, the function will return the first that has matched.
Template String :
Template literals are sometimes informally called template strings, because they are used most commonly for string interpolation (to create strings by doing substitution of placeholders). However, a tagged template literal may not result in a string; it can be used with a custom tag function to perform whatever operations you want on the different parts of the template literal.