Function decreasesByFunc

Export assertion functions

decreasesByFunc<R, T = any>(
    this: IAssertScope,
    getter: T | (() => any),
    prop?: keyof T,
    delta?: number | MsgSource,
    evalMsg?: MsgSource,
): R
Asserts that executing the target function decreases the monitored value by a specific delta. Unlike changesByFunc, the value must actually decrease. Note: The delta should be specified as a positive value representing the magnitude of the decrease. For example, if a value goes from 10 to 7, use delta of 3, not -3.
Type Parameters
- R
- T = any
Parameters
- this: IAssertScope
  The current IAssertScope object
- getter: T | (() => any)
  A function that returns a value to monitor, or an object containing the property to monitor
- Optionalprop: keyof T
  The property name to monitor (only used when first argument is an object)
- Optionaldelta: number | MsgSource
  The expected decrease amount (should be a positive number representing the magnitude)
- OptionalevalMsg: MsgSource
  Optional message to display if the assertion fails
Returns R
The current IAssertScope.that object
Example
```
let count = 10;
const getCount = () => count;
const subtractThree = () => { count -= 3; };

expect(subtractThree).to.decreaseBy(getCount, 3);  // Passes - decreased by 3 (use positive 3, not -3)
```
- Defined in assert/funcs/changesBy.ts:329