Unusual issue "Dictionary is potentially not defined" encountered in a codebase

Question

Unusual issue "Dictionary is potentially not defined" encountered in a codebase

Can you explain why the

stopsDict["first"].directions.push("test");

line is successful, but not the

stopsDict[stopName].directions.push("test");

one?

interface StopsDict {
  [key: string]: Stops;
}

interface Stops {
  directions?: string[];
}

let stopsDict: StopsDict = { 
  first: {
    directions: []
  },
  second: {}
};

if (Array.isArray(stopsDict["first"].directions)) {
  stopsDict["first"].directions.push("test"); //Operation Success
}

let stopName = "first";
if (Array.isArray(stopsDict[stopName].directions)) {
  stopsDict[stopName].directions.push("test"); //error TS2532: Object is possibly 'undefined'.
}

typescript

Answer 1

Answer №1

Due to the potential scenario where stopsDict[name].directions may be null, it's important to acknowledge this possibility. You can explicitly inform TypeScript that you are aware of this and take full responsibility for handling it:

stopsDict[name].directions!.push("test")

In a more complex scenario, it is advisable to implement checks to ensure that the conditions are met rather than solely relying on the type-checker.

For further information, refer to the documentation.

Notably, the following code snippet will not result in an error:

let dirs = stopsDict[stopName].directions;
if (Array.isArray(dirs)) {
  dirs.push("test");
}

Answer 2

Due to the potential scenario where stopsDict[name].directions may be null, it's important to acknowledge this possibility. You can explicitly inform TypeScript that you are aware of this and take full responsibility for handling it:

stopsDict[name].directions!.push("test")

In a more complex scenario, it is advisable to implement checks to ensure that the conditions are met rather than solely relying on the type-checker.

For further information, refer to the documentation.

Notably, the following code snippet will not result in an error:

let dirs = stopsDict[stopName].directions;
if (Array.isArray(dirs)) {
  dirs.push("test");
}

Unusual issue "Dictionary is potentially not defined" encountered in a codebase

Answer №1

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