Javascript：如何连接将未定义或空值视为空的 2 个值

Question

This is what I want:

var str = data + unit; // data:"2", unit: "rem", I want str: "223"
var str = data + unit; // data:"2", unit: null, I want  str: "2"
var str = data + unit; // data:"2", unit: undefined, I want  str: "2"
var str = data + unit; // data:undefined, unit: "rem", I want str: "rem"

Input and output values are given in the comments against each line. Normal concatenation appends "undefined" as a literal string. I want undefined and null values to be treated as empty string

Answer 1

You can use the nullish coalescing operator to convert null or undefined to an empty string.

var str = (data ?? "") + (unit ?? "");

For better browser support, you may want to use the logical or operator instead.

var str = (data || "") + (unit || "");

Answer 2

you can use a function

const checkValue = value => value ? value :  '';
console.log(checkValue(null) + checkValue(null)) // ''

Answer 3

You can check the unit before concatenation. If its value is either undefined or null set it to empty, or whatever you want. Then you can concatenate.

function concatenate(data, unit) {
    if(unit == undefined || unit == null) {
        unit = "";
    }
    return data + " " + unit;
}