Function changesByFunc

Asserts that executing the target function changes the monitored value by a specific delta. The sign of the delta is ignored - only the absolute value is compared.

Param: this

The current IAssertScope object

Param: targetOrFn

A function that returns a value to monitor, or an object containing the property to monitor

Param: propOrDelta

The property name to monitor (used when first argument is not an object) or the expected change amount

Param: delta

The expected change amount (sign is ignored, only absolute value matters)

Param: evalMsg

Optional message to display if the assertion fails

Returns

The current IAssertScope.that object

changesByFunc<R>(
    this: IAssertScope,
    func: () => any,
    delta: number,
    evalMsg?: MsgSource,
): R
Asserts that executing the target function changes the monitored value by a specific delta. Important: The sign of the delta is ignored - only the absolute value is compared. This means changeBy(5) and changeBy(-5) are equivalent and will both match a change of ±5.
Type Parameters
- R
Parameters
- this: IAssertScope
  The current IAssertScope object
- func: () => any
  A function that returns a value to monitor
- delta: number
  The expected change amount (sign is ignored, only absolute value matters)
- OptionalevalMsg: MsgSource
  Optional message to display if the assertion fails
Returns R
The current IAssertScope.that object
Example
```
let value = 10;
const getValue = () => value;
const addFive = () => { value += 5; };
const subtractFive = () => { value -= 5; };

expect(addFive).to.changeBy(getValue, 5);   // Passes - delta is +5
expect(addFive).to.changeBy(getValue, -5);  // Also passes - absolute value is 5
expect(subtractFive).to.changeBy(getValue, 5);  // Passes - absolute value is 5
expect(subtractFive).to.changeBy(getValue, -5); // Also passes - absolute value is 5
```
- Defined in assert/funcs/changesBy.ts:124
changesByFunc<R, T = any>(
    this: IAssertScope,
    target: T,
    prop: keyof T,
    delta: number | MsgSource,
    evalMsg?: MsgSource,
): R
Asserts that executing the target function changes the monitored value by a specific delta. Important: The sign of the delta is ignored - only the absolute value is compared. This means changeBy(5) and changeBy(-5) are equivalent and will both match a change of ±5.
Type Parameters
- R
- T = any
Parameters
- this: IAssertScope
  The current IAssertScope object
- target: T
  An object containing the property to monitor
- prop: keyof T
  The property name to monitor
- delta: number | MsgSource
  The expected change amount (sign is ignored, only absolute value matters)
- OptionalevalMsg: MsgSource
  Optional message to display if the assertion fails
Returns R
The current IAssertScope.that object
Example
```
const obj = { val: 10 };
const addFive = () => { obj.val += 5; };
const subtractFive = () => { obj.val -= 5; };

expect(addFive).to.changeBy(obj, 'val', 5);   // Passes - delta is +5
expect(addFive).to.changeBy(obj, 'val', -5);  // Also passes - absolute value is 5
expect(subtractFive).to.changeBy(obj, 'val', 5);  // Passes - absolute value is 5
```
- Defined in assert/funcs/changesBy.ts:147