哪一个是正确的？ 警报（（+“ 123”））警报（+“ 123”）警报（+（“ 123”））

Question

I'm trying to convert a string to a float. I know that parseFloat() can do that, but I've also found the syntax below, but without much reference.

What is the correct syntax, because they all seem to work. And where can I learn more about it? I don't know how to Google it, because I don't know what it's named.

// syntax 1
alert((+"123"));    // 123
alert((+"x123"));   // NaN
alert((+"123x"));   // NaN
alert((+"123   ")); // 123
alert((+"   123")); // 123
alert((+"12 3"));   // NaN

// syntax 2
alert(+"123");      // 123
alert(+"x123");     // NaN
alert(+"123x");     // NaN
alert(+"123   ");   // 123
alert(+"   123");   // 123
alert(+"12 3");     // NaN

// syntax 3
alert(+("123"));    // 123
alert(+("x123"));   // NaN
alert(+("123x"));   // NaN
alert(+("123   ")); // 123
alert(+("   123")); // 123
alert(+("12 3"));   // NaN

Answer 1

它们在语法上都是正确的...但是示例1和3具有多余的括号。

Answer 2

This is called implicit conversion. Since you used a mathematical operator (+), it tries to convert the string to a numeric value which is needed for mathematical operations. What you are asking here is give me the positive value of the following string.