Reactivity is a system at the core of the Lightning Web Components framework. The framework observes changes to the values of fields and properties. When it observes a change, it reacts. It reevaluates all the expressions used in the template and rerenders the component, which displays the new values.

Field and property are almost interchangeable terms. A component author declares fields in a class. An instance of the class has properties. To component consumers, fields are properties. In a Lightning web component, only fields that a component author decorates with @api are publicly available to consumers as object properties.

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