【JavaScript Advanced Program Design 4th Edition 】

Variable var const let

Variables increase the scope

Variables declared by const lets are not promoted in scope -> temporary dead zone Const lets cannot be declared repeatedly and are not an attribute of window objects const declaration restrictions apply only to references to variables to which they point

for (const key in {a:1, b:2}) {
    console.log(key)
}
// a, b
for (const key of [1, 2, 3, 4, 5]) {
    console.log(key)
}
// 1, 2, 3, 4, 5, 
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Statement style and best Practices:

1. Do not use the var
2. Const takes precedence over let

The data type

Data type judgment

7 basic data types: Undefined Null Boolean Number String Symbol Object

The typeof operator

• “undefiend”
• “boolean”
• “string”
• “number”
• “object”
• “function”
• “symbol”

type of null ?

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Undefined

Note: there is a difference between including undefined variables and undefined variables

let message; // let age console.log(message); // "undefined" console.log(age); / / an errorCopy the code

For undeclared variables, only one useful operation, Typeof, can be performed

Let message // Make sure this variable is not declared // let age console.log(typeof message); // "undefined" console.log(typeof age); // "undefined"Copy the code

Undefiant is a false value

Null

Null object Pointers are recommended to be initialized with NULL when defining the object value variable to hold in the future

typof null === 'object'
null == undefined
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Boolean

Number

Because using floating-point values uses twice as much memory as storing integers, ECMAScript always tries to convert values to the integer E

0.1 + 0.2 === 0.3?Copy the code

The range of values

• Number.MAX_VALUE
• Number.MIN_VALUE
• isFinite()
• Infinity
• -Infinity

NAN

IsNaN () type conversion

Not a Number

console.log(5/0); // 0
console.log(0/0); // NaN
console.log(-0/+0); // NaN
console.log(5/0); // Infinity
console.log(5/-0); // -Infinity
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Numerical transformation

Number() ParseInt() ParseFloat()

Number()

• Boolean values that convert true to 1 and false to 0
• Value, return directly
• Null, returns 0
• Undefined, returns NaN
• String, “string returns 0
• Object, the valueof() method, and converts the returned value according to the above rules. If the result is NaN, the toString () method is called and converted according to the conversion string method

parseInt(), parseFloat()

• Converts from the first non-space character
• If the first character is not a numeric character, add or subtract, and return NaN

String

Convert to string

• If the value has a toString() method, that method is called (with no arguments) and the result is returned
• null, “null”
• undefined, “undefined”

A numeric call to toString() can pass arguments