Javascript: How to concatenate 2 values treating undefined or null values as empty

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

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 || "");