(TypeScript2) How do you loop through an array of interface type?

Question

While using a for loop my let object has the type of string, even though the object I am iterating through is of a type defined in an interface.

Below is the code I am using. When trying to access mapping.attribute which is defined on the interface as a string, I get the error [Property 'attribute' does not exist on type 'string'.]

I have the following interface and function :

interface IMapping {
    attribute: string;
    property: string;
}

mapAttributes(mappings: IMapping[], values) {            
    for (let mapping in mappings) {
        if (mapping.hasOwnProperty("attribute")) {
            console.log(this.attributes.find(attribute => attribute.name === mapping.attribute).value);
        }
    }
}

How should the for loop be defined so that I can use a property that has been defined in my interface?

Answer 1

I was able to run your example when replacing

for (let mapping in mappings) {

with

for (let mapping of mappings) {

Answer 2

This occurred due to for..of vs. for..in statements

Both for..of and for..in statements iterate over lists; the values iterated on are different though, for..in returns a list of keys on the object being iterated, whereas for..of returns a list of values of the numeric properties of the object being iterated.

Here is an example that demonstrates this distinction:

let list = [4, 5, 6];

for (let i in list) {
   console.log(i); // "0", "1", "2",
}

for (let i of list) {
   console.log(i); // "4", "5", "6"
}