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class _Promise {
	constructor(exec) {
		_STATE_MAP = {
			FUlFILLED: 'FULFILLED',
			PADDING: 'PADDING',
			REJECT: 'REJECT'
		}
		_value = undefined
		_state = _STATE_MAP.PADDING
		_fulfilledFuncs = []
		_rejectFuncs = []
		exec(this.resolve.bind(this), this.reject.bind(this))
	}
	resolve(val) {
		if (this._state === this._STATE_MAP.PADDING) {
			this._state = this._STATE_MAP.FUlFILLED
			this._value = val
			this._fulfilledFuncs.map(it => it(val))
		}
	}
	reject(val) {
		if (this._state === this._STATE_MAP.PADDING) {
			this._state = this._STATE_MAP.REJECT
			this._value = val
			this._fulfilledFuncs.forEach(it => it(val))
		}
	}
	then(onFulfilled, onRejected) {
		if (this._state === this._STATE_MAP.PADDING) {
			onFulfilled instanceof Function
				? this._fulfilledFuncs.push(onFulfilled)
				: null
			onRejected instanceof Function ? this._rejectFuncs.push(onRejected) : null
		}
	}
}