Strict null checks for Map members

TypeScript Version: 2.0.0-beta Code

export interface INode {
    type: string;
    parentNode?: INode;
}

export interface IIdentifierNode extends INode {
    name: string;
}

public static isIdentifierNode (node: INode): node is IIdentifierNode {
    return node.type === NodeType.Identifier;
}

var namesMap = new Map<string, string>();

// main part
if (Nodes.isIdentifierNode(node) && namesMap.has(node.name)) {
    node.name = namesMap.get(node.name);  //`Type 'string | undefined' is not assignable to type 'string'.`
}

Expected behavior: No errors

Actual behavior: Type 'string | undefined' is not assignable to type 'string'.

Looks like here no TypeGuards for Maps?

[MoussaAdam]

any progress?

[use-strict]

This effectively forces users to add the postfix ! operator for every Map#get. It doesn’t look like something that can be statically analyzed by the compiler. There are two consequences, which to be honest, make me want to give up using strict null checks.

A. Type inference for return values is no longer reliable. Given that null or undefined can’t be automatically stripped in certain cases, like above, one can get an incorrect return type. Because the behavior is unpredictable, this means one has to always write the return type annotations and not use type inference. This is very inconvenient.

Example:

function getNumber(map: Map<string, number>, key: string, defaultValue: number) {
    if (map.has(key)) {
        return map.get(key);
    }
    return defaultValue;
}

The inferred return type is number|undefined, even though it can never be undefined.

B. The explicit ! acts just as a type assertion so it suffers from the same limitations. It’s an unconditional bail from the compiler checks. Since there is no way to relate it to the condition (Map#has in this case), once the condition is changed, the user also has to revisit the !. This is just as error-prone as not having strict null checks and remembering to check against null values.