Javascript

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The notes about javascript

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data structure

• string immutable
• array mutable
• Non primitive data types are mutable data types that are not undefined, null, boolean, number, string, or symbol. Mutable means that the data can be changed after it is created.

  variable names use camel case

• let define the variable but variabe can change anytime
• const define the variable but the variable cant change anytime
• • const define a variable must have the value not the undefined

    array

• push()
  • push() returns the new length of the array, after adding the value you give it.
• pop()
  • delete the last element and return the value;
  • unshift() -The .unshift() method of an array allows you to add a value to the beginning of the array,and return the length of the array.
• shift()
  • Arrays also have a .shift() method. This will remove the first element of the array

    string

• sting.repeat() repeat the string
  • This method accepts a number as an argument, specifying the number of times to repeat the target string

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scope

• Variables in JavaScript are available in a specific scope. In other words, where a variable is declared determines where in your code it can be used.
• The first scope is the global scope. Variables that are declared outside of any "block" like a function or for loop are in the global scope
• A variable declared inside a function can only be used inside that function
• An important thing to know about the return keyword is that it does not just define a value to be returned from your function, it also stops the execution of your function code.
• Returning a value from a function brings that value into the scope where the function was called.

  Use the space " " to occupy the space

  Boolean

• JavaScript has a defined list of falsy values. Some of them include false, 0, "", null, undefined, and NaN.

  "=="

• "0" == 0 is true because the data structure can change automatic

  "==="

• The strict equality operator === is used to check if two values are equal and share the same type.
• this is the equality operator you should always use

  "!=="

• The strict inequality operator !== allows you to check if two values are not equal, or do not have the same type.

  The logical OR operator

• The logical OR operator will use the first value if it is truthy – that is, anything apart from NaN, null, undefined, 0, -0, 0n, "", and false. Otherwise, it will use the second value.

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DOM

• queryselector()
  • The querySelector() method takes a CSS selector as an argument and returns the first element that matches that selector.

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ternary operator

• The ternary operator is a conditional operator and can be used as a one-line if-else statement. The syntax is: condition ? expressionIfTrue : expressionIfFalse.
• The first is the condition.

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string methods

• str.replace()
  • Since strings are immutable, the replace method returns a new string with the replaced characters.