Variable declaration and type checking

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1. Variable declarations and types

Var let const difference

1. Var is ES5 syntax, let and const are ES6 syntax
2. Var has a variable boost
3. Var and let are variables that can be modified. Const is a constant and cannot be modified (it can be used to define reference types)
4. Let, const block-level scope; Var function scope

【ES6】 Variable declaration -var-let-const- Distinction and connection – summary

1.2 Data Types

• Value types: Undefined, Null, Number, String, Boolean, Symbol(ES6), BigInt(ES10)
• Reference types: Object: Array, Function

Data types – Basic data types – Reference data types – Conversion between types – Determination of data types

1.3 The difference between value types and reference types

• The value type is stored in stack memory, and the variable gets its value
• The reference type is stored in the heap, and all the variable gets is a reference to it, its address

【JS】JavaScript- Object- Object- Built-in Object- Host Object- Custom Object- Operation Object- Basic data type and reference data type difference

1.4 TypeOF Determines the types

1. Undefined, string, number, Boolean, symbol, bigint
2. Function 【 function 】
3. Object, (typeof null === ‘object’)

1.5 Method for determining data types

1. Typeof [other than null basic type + function]
2. Instanceof [reference type] [from subclass to superclass to object]
3. ToString () [any type]
4. Array.isarray ()

1.6 Implement a general function to determine data types

const getType = (s) = > {
  const r = Object.prototype.toString.call(s)

  return r.replace(/\[object (.*?)\]/.'$1').toLowerCase()
}

/ / test
console.log(getType()) // undefined
console.log(getType(null)) // null
console.log(getType(1)) // number
console.log(getType('YK bacteria')) // string
console.log(getType(true)) // boolean
console.log(getType(Symbol('YK bacteria'))) // symbol
console.log(getType({})) // object
console.log(getType([])) // array
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1.7 = = = 与 = =

=== strictly compares equality

== does a cast and then a comparison

The following is true

100= ='100'
0= =' '
0= =false
fase == ' '
null= =undefined
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Here’s a case where we can use theta = theta

if(a == null) {}
/ / equivalent to the
if(a === null || a === undefined) ()Copy the code

1.8 TRULY variables and Services variables

• Truly variables:!!!!! a === trueThe variables of
• Falsely variables:!!!!! b === falseThe variables of

Falsey variables are listed below, and all but six of them are Truely variables

!!!!!0= = =false!!!!!NaN= = =false!!!!!' '= = =false!!!!!null= = =false!!!!!undefined= = =false!!!!!false= = =false
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1.9 Casting and Implicit Casting

• Mandatory:parseInt,parseFloat,toString
• Implicit:if, logical operation,= =,+Concatenated string

ES5 datatypes – Basic datatypes – Reference datatypes – Conversion between types – Datatypes

1.10 Statements and Expressions

Expression: An expression produces a value, which can be placed wherever a value is needed

a
a+b
demo(1)
x===y? 'a': 'b'
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statements

if() {}for() {}Copy the code

Finally, welcome to pay attention to my column and make good friends with YK bacteria!