How to use the jsbi.subtract function in jsbi

it('< MIN_SAFE_INTEGER: IntSize.BigInt', () => {
            let value = JSBI.subtract(JSBI.BigInt(Number.MIN_SAFE_INTEGER), JSBI.BigInt(1));
            let reader = ion.makeReader(intToBinaryIonBytes(value));
            assert.equal(IonTypes.INT, reader.next());
            assert.equal(reader.intSize(), IntSize.BigInt);
        });
    });

const a = JSBI.BigInt(Number.MAX_SAFE_INTEGER);
const b = JSBI.BigInt(12);
console.log(JSBI.add(a, b));
JSBI.BigInt('1');
JSBI.BigInt('34');
JSBI.BigInt('00034');

const JSBigInt = JSBI.BigInt;
JSBigInt(34);
const JSBigInt2 = JSBI['BigInt'];
JSBigInt2(56);
const JSBIadd = JSBI.add;
JSBIadd(7, 8);

JSBI.add(a, b);
JSBI.subtract(a, b);
JSBI.multiply(a, b);
JSBI.divide(a, b);
JSBI.remainder(a, b);
JSBI.exponentiate(a, b);
const c = JSBI.unaryMinus(a);
const d = JSBI.bitwiseNot(a);
JSBI.leftShift(a, b);
JSBI.signedRightShift(a, b);
JSBI.bitwiseAnd(a, b);
JSBI.bitwiseOr(a, b);
JSBI.bitwiseXor(a, b);

JSBI.equal(a, b);
JSBI.notEqual(a, b);
JSBI.lessThan(a, b);
JSBI.lessThanOrEqual(a, b);

it('< MIN_SAFE_INTEGER: IntSize.BigInt', () => {
            let value = JSBI.subtract(JSBI.BigInt(Number.MIN_SAFE_INTEGER), JSBI.BigInt(1));
            let reader = ion.makeReader(value.toString());
            assert.equal(IonTypes.INT, reader.next());
            assert.equal(reader.intSize(), IntSize.BigInt);
        });
    });

private static calculateSizeThreshold(numberOfBytes: number): JSBI {
        let exponent: JSBI = JSBI.multiply(JSBI.BigInt(numberOfBytes), this.BITS_PER_BYTE);
        let threshold = JSBI.exponentiate(JsbiSupport.TWO, exponent);
        return JSBI.subtract(threshold, JsbiSupport.ONE);
    }
}