Fair Value Measurement (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Summary of Assets and Liabilities that Require Fair Value Measurements on a Recurring Basis

The following table summarizes the Company’s assets and liabilities that require fair value measurements on a recurring basis and their respective input levels based on the fair value hierarchy (in thousands):

			Fair Value Measurements at Reporting Date
	Total		Level 1		Level 2		Level 3
As of December 31, 2018:
Assets
Cash equivalents	$	59,357	$	59,357	$	—	$	—
As of December 31, 2017:
Assets
Cash equivalents	$	12,472	$	12,472	$	—	$	—
Liabilities
Derivative liability	$	3,028	$	—	$	—	$	3,028
Preferred stock warrant liability		1,656		—		—		1,656
Total liabilities	$	4,684	$	—	$	—	$	4,684

Weighted-Average Assumptions Used In Determining Fair Value of Preferred Stock Warrant Liability Valued Using Black-Scholes Option Pricing Model

The following weighted-average assumptions were used in determining the fair value of the preferred stock warrant liability valued using the Black-Scholes option pricing model as of December 31, 2017:

	December 31, 2017
Expected volatility		68.0	%
Risk-free interest rate		2.2	%
Contractual term (in years)		5.9
Expected dividend yield		—

Estimated Fair Values Per Share of Underlying Redeemable Convertible Preferred Stock

The following estimated fair values per share of the Company’s underlying redeemable convertible preferred stock were used to determine the estimated fair value of the preferred stock warrant liability:

	December 31, 2017
Series AA	$	5.75
Series B	$	3.76
Series C-1	$	3.76

Activity for Preferred Stock Warrant Liability and Derivative Liability Measured At Fair Value Using Significant Unobservable Level 3 Inputs

The following tables present activity for the preferred stock warrant liability and the derivative liability measured at fair value using significant unobservable Level 3 inputs during the year ended December 31, 2017 and 2018 (in thousands):

	Preferred Stock
	Warrant Liability
Balance at December 31, 2016	$	880
Issuance of warrants		1,795
Changes in fair value reflected as change in fair    value of warrant liability		(1,019	)
Balance at December 31, 2017		1,656
Issuance of warrants		2,822
Exercise of warrants		(1,197	)
Changes in fair value reflected as change in fair    value of warrant liability		(11	)
Conversion to common stock warrants upon    IPO		(3,270	)
Balance at December 31, 2018	$	—

	Derivative Liability
Balance at December 31, 2016	$	—
Issuance of derivative		2,971
Changes in fair value reflected as change in    fair value of derivative liability		57
Balance at December 31, 2017		3,028
Changes in fair value reflected as change in    fair value of derivative liability		676
Extinguishment of derivative upon conversion    of convertible promissory notes		(3,704	)
Balance at December 31, 2018	$	—