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Reason: data that is not wrapped in return will be visible globally in the project, resulting in variable contamination, while variables in the data wrapped with return will only take effect in the current component and will not affect other components.

The operating environment of this tutorial: windows7 system, vue2.9.6 version, DELL G3 computer.

Official:

When a component is defined, the data must be declared as a function that returns an initial data object, because the component may be used to create multiple instances. If data

If it is still a pure object, all instances will share and reference the same data object! By providing the data function, we can call data every time we create a new instance

Function to return a completely new copy of the original data object.

Why does data need to use return to return data in a project?

Data that is not wrapped with return will be visible globally in the project and will cause variable contamination; variables in the data wrapped with return will only take effect in the current component and will not affect other components.

When a component is defined, the data must be declared as a function that returns an initial data object, because the component may be used to create multiple instances. If data

If it is still a pure object, all instances will share and reference the same data object! By providing the data function, each time a new instance is created, we can call the data function to return a new copy of the original data object.

Analogies and reference data types. If we don't use function return, the data of each component is the same address of memory, that one data changes and the other changes, which is certainly not what we want. Using function return is tantamount to declaring new variables, independent of each other, so there will be no such problem; js assigns object objects to the same memory address directly. So this approach is adopted for the independence of the data of each component.

If it is not a component, the normal data can be written as an object directly. For example, two calculators in the same component share data: {num: 0}. Sharing objects directly in JS will cause reference passing, that is, all numb values will be changed after the add and subtract button is pressed, so function is used here to return one object instance at a time.

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