The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type

1    interface Dimensions {
2        width: Number,
3        height: Number
4    }
5
6    function findArea(dimensions: Dimensions): Number {    
7        return dimensions.height * dimensions.width;
8    }

line 7, red squiggly lines under dimensions.height and dimensions.width

> The left-hand side of an arithmetic operation must be of type 'any', > 'number' or an enum type.

I'm trying to eradicate red squiggly, but I'm stumped as to why the typescript compiler is giving me an error. As far as I can tell, width and height are of type Number.

Here is another example of this error occurring that might help people.

If you're using typescript and trying to compute the difference between dates (In my case I was attempting to use ISO string from database):

new Date("2020-03-15T00:47:38.813Z") - new Date("2020-03-15T00:47:24.676Z")

TypeScript Playground

It will show the error.

However, this same exact code works if you put it in the browser console or other node environment

new Date("2020-03-15T00:47:38.813Z") - new Date("2020-03-15T00:47:24.676Z")
14137

I may be wrong, but I believe this works due to the - operator implicitly using valueOf on each of it's operands. Since valueOf on Date returns a number the operation works.

However, typescript doesn't like it. Maybe there is a compiler option for this is forcing this constrain and I'm not aware.

new Date().valueOf()
1584233296463

You can fix by explicitly making the operands number (bigint) types so the - works.

Fixed Example

new Date("2020-03-15T00:47:38.813Z").valueOf() - new Date("2020-03-15T00:47:24.676Z").valueOf()

TypeScript Playground

Number should be lowercase: number.

• See sample on Playground

• Documentation for basic types

cleanest way I found:

const diff = +new Date("2020-03-15") - +new Date("2020-03-15")

https://github.com/microsoft/TypeScript/issues/5710

you can cast the expression to number.

function findArea(dimensions: Dimensions): Number {    
    return Number(dimensions.height) * Number(dimensions.width);
}

> ERROR in src/app/demo.component.ts(...): error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.

package.json -

"dependencies": {
    "@angular/core": "^6.1.10"
}

"devDependencies": {
    "typescript": "^2.9.2"
}

Code -

dateISOString(d: any): string {
    var nDate = new Date(d);
    var tzoffset = nDate.getTimezoneOffset() * 60000;
    var localISOTime = (new Date(nDate - tzoffset)).toISOString(); // This line gives error
    return localISOTime;
}

Solution - Add valueOf() with date

var localISOTime = (new Date(nDate.valueOf() - tzoffset.valueOf())).toISOString();

I ran into this issue when subtracting two dates, the error was bugging me but the program still outputted as expected. Below is code which is sorting an object array by date (newest to oldest).

Issue

var Sorted = Data.sort(function(a,b){ return new Date(b.Posted) - new Date(a.Posted) })

Solution

var Sorted = Data.sort(function(a,b){ return new Date(b.Posted).getTime() - new Date(a.Posted).getTime() })

• Dates: append each value .getTime() to convert from date to number.
• Numbers: wrap each value with parseInt()

From my understanding, this error won't stop the compiler/program from executing; however it's better practice to provide your development environment with explicit/controlled variable types.

For future reference; Javscript & Typescript can be unpredictable when it comes to comparing/calculating certain variables, so using explicit types and casting variables will remove these warnings/errors.

I got to find out that this issue is called by using a .ts file extension instead of .tsx file extension. I recently ran into the issue. its solved my problem