swc/crates/swc_ecma_parser/tests/tsc/additionOperatorWithNullValueAndValidOperator.ts

// If one operand is the null or undefined value, it is treated as having the type of the other operand.

enum E { a, b, c }

var a: any;
var b: number;
var c: E;
var d: string;

// null + any
var r1: any = null + a;
var r2: any = a + null;

// null + number/enum
var r3 = null + b;
var r4 = null + 1;
var r5 = null + c;
var r6 = null + E.a;
var r7 = null + E['a'];
var r8 = b + null;
var r9 = 1 + null;
var r10 = c + null
var r11 = E.a + null;
var r12 = E['a'] + null;

// null + string
var r13 = null + d;
var r14 = null + '';
var r15 = d + null;
var r16 = '' + null;