Array type in type script

Question

Why is this allowed by TypeScript? I specified a numeric index. Why can I use a string as an index? Visual studio doesn\'t report an error.

interface StringA         


        

Answer 1

Problem

It's because the compiler is still allowing implicit any types which can happen when accessing a property of an object by using an index:

// Example 1
let dictionary: { [index: number]: string };
let myStringTypedVar = dictionary[5];   // implicitly typed as "string"
let myAnyTypedVar = dictionary["prop"]; // implicitly typed as "any"

// Example 2
let myNumberTypedVar = 5;
let myAnyTypedVar = myNumberTypedVar["prop"]; // implicitly typed as "any"

Fix: Compile with --noImplictAny

If you compile your example with --noImplictAny then it will error:

tsc --noImplicitAny example.ts

Outputs:

example.ts(8,9): error TS7017: Index signature of object type implicitly has an 'any' type.

I would recommend always compiling with --noImplicitAny. In Visual Studio, you can turn on --noImplictAny by unchecking "Allow implicit 'any' types" in the project properties' typescript build tab:

Or by adding "noImplicitAny": "true" to compilerOptions in tsconfig.json.

Answer 2

An array is also an objects. So you can access the object properties.

var array = [];
array.push("One"); // array
array[1]= "Two"; // array
array['key'] = "Three";// object
array.key2 = "Four"; // object
var length = array.length; // Is 2 

Answer 3

A numeric index signature only defines the type for properties accessed through a numeric index. It does not restrict property access to only a numeric index.

From the TypeScript 1.5 specification:

Numeric index signatures, specified using index type number, define type constraints for all numerically named properties in the containing type. Specifically, in a type with a numeric index signature of type T, all numerically named properties must have types that are assignable to T

But, I think you have a good point. It does seem like properties should not be accessed by a string index if you have only defined a numeric index signature.