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Js2day (also i++ and ++i, if statements, ternary operators, switch, while statements, for loop statements)

2022-07-02 12:45:00 【By the Difeng River】

List of articles

again i++ and ++i

let i = 1;
alert(i++ + ++i + i)  //7

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Comparison operator

Use of comparison operators
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        alert(5 == "5")    //true
        alert(5 === "5")    //false
        alert(5 !== "5")    //true

String comparison , Is the character corresponding to the comparison ASCII code
Compare from left to right
If the first is the same, compare the second , And so on
Relatively few , Understanding can
NaN Not equal to any value , Include It itself
As far as possible Don't compare decimals , Because there is a precision problem with decimals
Implicit conversion occurs when comparing different types
Finally, the data is implicitly transformed into number Compare the types again
So in development , If we make an accurate comparison, we prefer === perhaps !==

if sentence

Pay attention to the use of parentheses .
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Ternary operator

Judge the maximum value of two numbers

    let first = +prompt(" First number ：")
    let second = +prompt(" The second number ：")
    alert(first > second ? first : second)

Be careful ： there prompt Ahead + No. is very necessary , Implicitly convert to Number type .（ Otherwise, enter 5,10 The maximum output result is 5, the reason being that String comparison ）

switch sentence

Be careful ：switch The data in should be consistent with case The value in is Congruent
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while sentence

        //while Simple circulation ATM machine 
        let total = 0;    // It's defined in script Global variables in the block level 
        let money = 0;    // It's defined in script Global variables in the block level 
        while (true) {
    
            let i = +prompt(` Please select your operation ： 1. deposit  2. Withdraw money  3. Check the balance  4. sign out `)
            if (i === 4) {
    
                break   // Jump out of while loop 
            }
            switch (i) {
    
                case 1:
                    money = +prompt(" Please enter your deposit amount ：")
                    total += money
                    break  	// Jump out of switch loop 
                case 2:
                    money = +prompt(" Please enter your withdrawal amount ：")
                    total -= money
                    break	// Jump out of switch loop 
                case 3:
                    alert(total)
            }
        }

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for Loop statement multiplication table

    <style>
        div {
    
            display: inline-block;
            height: 25px;
            line-height: 25px;
            margin: 5px;
            background-color: pink;
            padding: 0 10px;
            border: 1px solid hotpink;
            color: deeppink;
            border-radius: 5px;
            box-shadow: 2px 2px 2px rgba(0, 0, 0, .2);
            text-align: center;
        }
    </style>
</head>

<body>
    <script>
        //  Print a few lines on the outer layer 
        for (let i = 1; i <= 9; i++) {
    
            //  Print a few stars on the inner layer 
            for (let j = 1; j <= i; j++) {
    
                //  Just need it  *  Switch to  1 x 1 = 1 
                document.write(`<div> ${
      j} x ${
      i} = ${
      j * i} </div>`)
            }
            document.write('<br>')
        }
    </script>
</body>