以前的所有答案都不错，但这会更好.使用双重NOT运算符(!!):

All the previous answers are good, but this will be even better. Use dual NOT operators (!!):

if (!!str) {
    // Some code here
}

或使用类型转换:

if (Boolean(str)) {
    // Code here
}

两者都具有相同的功能.将变量转换为布尔型，其中str是变量.
它为null，undefined，0，000，""，false返回false.
它为字符串"0"和空格" "返回true.

Both do the same function. Typecast the variable to Boolean, where str is a variable.
It returns false for null, undefined, 0, 000, "", false.
It returns true for string "0" and whitespace " ".