Typescript type error doesn't make sense

Question

interface A
  {
    x:string; y:string
  };

var a: A[] = [];
a.push(
  {x: "a", 
   y: "b"});

a.slice(-1).x = "foo";

That last line gets an error I think because the result of a.slice(-1) is not of a known type, so it says "no known property x".

Is my diagnosis correct?
What's the right way to do that?

From the official close reasons: Questions seeking debugging help ("why isn't this code working?") must include the desired behavior, a specific problem or error and the shortest code necessary to reproduce it in the question itself. — halfer
– halfer, Commented Mar 25, 2018 at 10:57

basarat · Accepted Answer · 2015-07-07 23:14:36Z

0

The problem is that a.slice(-1) returns an array (the slice) even though it has only one element. An array does not have the "x" property. But each element does.

So the following does what is expected:

a.slice(-1)[0].x = "foo";

edited Jul 7, 2015 at 23:14

basarat

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answered Jul 7, 2015 at 19:25

pitosalas

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2 Comments

basarat Over a year ago

TypeScript saving ya ;)

pitosalas Over a year ago

yeah, but it could have given a more useful error message, like: value typeof "array" does not have property "x".

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