Create History State

Task

Add a history collection to the calculator state.

Open

Open:

js/calculator.js

Add history to the state created in the constructor.

history: []

Then add a method for creating a history item:

addHistory() {
  const { lastExpression, current } = this.state;

  if (!lastExpression || current === 'Error') {
    return;
  }

  this.state.history.unshift({
    expression: lastExpression,
    result: current,
    timestamp: Date.now()
  });
}

Use unshift() so the newest calculation appears first.

Update equals

At the end of a successful equals() calculation, call:

this.addHistory();

Place it after lastExpression and current have been updated.

Test

Run:

calculator.clear();
calculator.inputDigit('8');
calculator.chooseOperation('+');
calculator.inputDigit('2');
calculator.equals();
calculator.state.history;

You should see one history item containing the expression, result, and timestamp.

Checkpoint

Completed calculations are now stored in application state.