The confusion comes up when you do a _shallow_ copy: Some modifications will modify the original variable, and other modifications won't. Pass-reference-by-val

The confusion comes up when you do a _shallow_ copy: Some modifications will modify the original variable, and other modifications won't. Pass-reference-by-value is just a special case of a shallow copy. let a = [1, 2, 3]; let b = a; b[1] = 5; // will modify a b = [1, 6, 3]; // will not modify a In languages like C++, there's a way to make it so that last line _would_ modify a, which is why there's a need to use different words for the different phenomenon.