JavaScript compares the size of two numbers

1. There are three situations for comparing the size of two numbers in JS:

1. Comparing two pure numbers (Number): normal mathematical operation.

2. Compare the size of pure numbers (Number) and string numbers (String): automatically convert string numbers into pure numbers, and then perform comparisons between pure numbers

3. Compare the size of the string number (String) with the string number (String): it will be compared according to the ASCII code of the first different character

console.log( 21 > 3 );     // true
console.log( '21' > 3 );   // true
console.log( '21' > '3' ); // false
console.log( Number('21') > Number('3') ); // true

Therefore, when comparing numbers, pay attention to converting strings into numbers before comparing.

2. Compare the size of super long numbers

            // 在JS中会存在数字大小操作2的53次方时会精度丢失，末尾几位变成0的情况。
            // JS数字安全长度16位及以下，
            console.log( '16位',  1234567891234567 );    // 1234567891234567
            console.log( '17位',  12345678912345678 );   // 12345678912345678
            console.log( '18位',  123456789123456789 );  // 123456789123456780
            console.log( '19位',  1234567891234567891 ); // 1234567891234568000
            // 精度丢失就会存在数字比较大小不准确的问题，比如：
            let a = '1234567891234567891';
            let b = '1234567891234567899';
            console.log( Number(a) < Number(b) ) // false
            // 超过16位的正整数比较大小的方法:先转成字符串再截取比较大小
            if (a.length > b.length) {
                console.log('a > b');
            } else if (a.length < b.length) {
                console.log('a < b');
            } else {
                const a1 = a.substr(0, 16);
                const a2 = a.substr(16, a.length-16);
                const b1 = b.substr(0, 16);
                const b2 = b.substr(16, b.length-16);
                if (Number(a1) > Number(b1) || (Number(a1) === Number(b1) && Number(a2) > Number(b2))) {
                    console.log('a > b');
                } else if (Number(a1) < Number(b1) || (Number(a1) === Number(b1) && Number(a2) < Number(b2))) {
                    console.log('a < b');
                } else {
                    console.log('a = b');
                }
            }
            // 最后输出结果：a < b