In Typescript, what is the ! (exclamation mark / bang) operator when dereferencing a member?

Question

When looking at the sourcecode for a tslint rule, I came across the following statement:

if (node.parent!.kind === ts.SyntaxKind.ObjectLiteralExpression) {
    return;
}

Notice the ! operator after node.parent. Interesting!

I first tried compiling the file locally with my currently installed version of TS (1.5.3). The resulting error pointed to the exact location of the bang:

$ tsc --noImplicitAny memberAccessRule.ts 
noPublicModifierRule.ts(57,24): error TS1005: ')' expected.

Next I upgraded to the latest TS (2.1.6), which compiled it without issue. So it seems to be feature of TS 2.x. But the transpilation ignored the bang completely, resulting in the following JS:

if (node.parent.kind === ts.SyntaxKind.ObjectLiteralExpression) {
    return;
}

My Google fu has thus far failed me.

What is TS's exclamation mark operator, and how does it work?

Answer 1

That's the non-null assertion operator. It is a way to tell the compiler "this expression cannot be null or undefined here, so don't complain about the possibility of it being null or undefined." Sometimes the type checker is unable to make that determination itself.

It is explained here:

A new ! post-fix expression operator may be used to assert that its operand is non-null and non-undefined in contexts where the type checker is unable to conclude that fact. Specifically, the operation x! produces a value of the type of x with null and undefined excluded. Similar to type assertions of the forms <T>x and x as T, the ! non-null assertion operator is simply removed in the emitted JavaScript code.

I find the use of the term "assert" a bit misleading in that explanation. It is "assert" in the sense that the developer is asserting it, not in the sense that a test is going to be performed. The last line indeed indicates that it results in no JavaScript code being emitted.