Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Total Stock-Based Compensation Expense Recognized

Total stock-based compensation expense recognized is as follows (in thousands):

	Year Ended December 31,
	2018		2017		2016
Research and development	$	1,792	$	175	$	108
General and administrative		2,778		227		74
Total	$	4,570	$	402	$	182

Summary of Weighted-Average Assumptions Used to Calculate Fair Value of Stock-based Awards Granted

The following weighted-average assumptions were used to calculate the fair value of stock-based awards granted to employees and directors during the periods indicated:

	Year Ended December 31,
	2018			2017			2016
Risk-free interest rate		2.79	%		1.83	%		1.64	%
Expected volatility		73.47	%		77.59	%		73.22	%
Expected dividend yield		—			—			—
Expected term (in years)		6.01			6.08			6.02

Summary of Stock Option Activity

Activity under the 2018 Plan and the 2012 Plan is summarized as follows (in thousands, except per share data):

				Weighted-		Weighted-
				Average		Average		Aggregate
	Options			Exercise		Remaining		Intrinsic
	Outstanding			Price		Years		Value
Balance at December 31, 2017		4,884		$	0.67		8.7	$	16,331
Granted		3,789			17.19
Exercised		(531	)		0.65
Forfeited		(331	)		2.62
Balance at December 31, 2018		7,811		$	8.60		8.2	$	342,292
Options exercisable		3,684		$	1.02		7.0	$	188,812
Options vested and expected to vest		7,770		$	8.57		8.2	$	340,773

Summary of Weighted-Average Assumptions Used to Calculate Fair Value of ESPP Shares

The following weighted-average assumptions were used to calculate the fair value of ESPP shares during the year ended December 31, 2018:

Risk-free interest rate	2.42	%
Expected volatility	65.92	%
Expected dividend yield	—
Expected term (in years)	1.24

Summary of Restricted Common Stock Activity

A summary of the restricted common stock activity during the year ended December 31, 2018 is as follows (in thousands, except per share data):

				Weighted-
				Average
	Number			Grant Date
	of Shares			Fair Value
Balance at December 31, 2017		104		$	0.43
Vested		(57	)	$	0.43
Repurchased		—		$	—
Balance at December 31, 2018		47		$	0.43