Typescript: Pass an extra parameter

[nirus]

I was trying the below code from the documentation

interface Point { 
 x: number; 
 y: number; 
}

function getX(p: Point) { 
 return p.x; 
}

class CPoint { 
 x: number; 
 y: number; 
 constructor(x: number, y: number) { 
 this.x = x; 
 this.y = y; 
 } 
}

getX(new CPoint(0, 0)); // Ok, fields match

getX({ x: 0, y: 0, color: "red" }); // Extra fields Ok

getX({ x: 0 }); // Error: supplied parameter does not match

As per the code comment says below line should be ok.

getX({ x: 0, y: 0, color: "red" }); // Extra fields Ok

But i am getting error as below:

error TS2345: Argument of type '{ x: number; y: number; color: string; }' is not assignable to parameter of type 'Point'. Object literal may only specify known properties, and 'color' does not exist in type 'Point'

But the below code works well which i re-wrote in which i made params as optional:

interface Point { 
 x: number; 
 y?: number; 
 color?: string; 
}

function getX(p: Point) { 
 return p.x; 
}

class CPoint { 
 x: number; 
 y: number; 
 constructor(x: number, y: number) { 
 this.x = x; 
 this.y = y; 
 } 
}

getX(new CPoint(0, 0)); // Ok, fields match

getX({ x: 0, y: 0, color: "red" }); // Extra fields Ok

getX({ x: 0 }); // Error: supplied parameter does not match

Please can somebody help me out if the documentation is wrong or am i missing something here

FYI i am using:

• Typescript v1.7.5
• Visual studio code

Same is been asked in stackoverflow:

I have attached the screenshot FYI.

[yortus]

I'd say the documentation is wrong / out of date. The error you are seeing is an example of #3823. It would have worked before that PR, which landed in v1.6.

[DanielRosenwasser]

This is in the spec, not the docs, just for tagging clarification.

[nirus]

As pointed by the Stackoverflow answer below code fixes the problem

getX({ x: 0, y: 0, color: "red" } as Point); // no error

This is not given in spec. Is this a feature or the implementation has been changed as a part of progression.

[weswigham]

It's a cast - casting is in the spec, and it's a feature. Just remember that a cast is basically you overriding the compiler and saying "this is the type of this thing at this position", and so its mostly your responsibility to make sure it's valid to cast in a given situation.

[weswigham]

A "nicer" way to "fix" the types and allow passing an argument with extra members would be modifying the signature of getX like so:

function getX<T extends Point>(p: T) { 
 return p.x; 
}

Though it may not be immediately obvious how this is any different, this alters inference to allow for any subtype of Point to be passed as an argument, bypassing the "strictness" check added in 1.6.

[mhegazy]

for future references, more information about strict object literal checking can be found at https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#strict-object-literal-assignment-checking